Fibre connection magically working?

Hi all, this is my first post.

I'm reaching out to the community with my NZ UFB related questions due to finding it difficult to get detailed information about the network parameters from either Chorus or my ISP.

Context:

* Having had no personal internet connection since Dialup, I'm completely out of the loop and have ancient equipment

* I have acquired various arbitrary routers from family or otherwise because I collect things like that in the rare event they're useful

* This year, I got ADSL for the first time to make work from home bearable during COVID-19

* Fibre was not an option at the time, as it would have been unlikely to get a fibre connection during lockdown

* This month, since finding ADSL to be somewhat slow (marginally better than the 3G/4G service I had on my phone - mainly just heaps more data), and Chorus begging me to install fibre at my place, I decided to go ahead and switch to fibre

Then the rollercoaster ride began!

So when I signed up with my ISP I asked to have them send me their router as I wanted no trouble. I was working from home again, and wanted the connection ASAP. Given that the fibre was in the ground and the ONT on the wall on the 17th, I (and my ISP) thought I'd have an active connection within 2-3 days.

Alas, there was a miscommunication/timing issue between installing fibre and ordering the connection through my ISP which resulted in me not getting connected for about 10 working days. Also, my ISP seemingly forgot to send my router, so I had a couple of days of active connection but unable to use it (ADSL fallback sigh).

When my connection was first activated, since I still had no router, I decided to plug my PC direct to the ONT because I run Linux and I figured I could perhaps configure it properly. To my surprise, internet worked out-of-the-box with a direct connection to the ONT!

First question: How is this possible? I didn't configure VLAN or PPPoE or anything....

I disconnected my PC from the ONT though due to a personal concern that my default firewall setup may not be secure - I think I also had enabled SSH which is probably not the best idea for a public facing machine.

I decided to stick to the painfully unusable ADSL connection until the router from my ISP arrived.

Turns out, the router from my ISP never came so I decided to take the risk and plug my PC back directly to the ONT - so I could at least work with out issues. But, mysteriously, the connection no longer magically worked. So I went and bought a router. I specifically looked out for one that supports VLAN tagging as I thought this was required. I figured that there may have been a brief moment where the network was misconfigured at ISP side and so my PC could connect no issue. I assumed that this had been rectified and now I'd require a properly configured router.

After some time trying to get my ISP to divulge the network parameters (honestly, I don't think they actually know themselves), I managed to get internet working with the new router I'd purchased. And that was that...

But now that it's Sunday and I have time to try some things, I decided to try using my ADSL router's LAN port - just in case it worked. To my surprise, this did work! So I'm thinking to return the new router I purchased as it seems unnecessary.

So I called my ISP and asked how it's possible that my old router (I'm almost 100% certain it does NOT support VLAN tagging) just magically seems to work. They didn't seem to know, but also didn't seem to think it should matter if there are VLANs or not. One important thing to note at this time is that they discovered my ONT is NOT tagging the traffic when it should be. So they put a request to their provisioning team to fix that.

Another important thing to note, is that I had suspicions that direct PC connection would also work again now, given my old router was now working and it was configured to use IPoE (not PPPoE). Yes, direct PC connection to ONT was working again.

My second question: is it possible that things are working now only because my ONT is not yet tagging traffic? ie, I'm getting lucky? Is it possible that this will all break once my ISP/Chorus configures my ONT to tag the traffic?

Lastly, it sounds like this whole VLAN thing really doesn't matter.... unless perhaps you have a VoIP phone (I don't have a landline so no need for VoIP over fibre). Does anyone know the real deal about whether VLAN tagging is actually necessary? Or is it just for QoS?

Thanks so much for your time. I hope someone here can help clear up what's going on.

NOTE: I hold a CCNA (expired) so I have at least rudimentary knowledge of the full network stack, though I confess I am no expert and only know the theory of VLAN, TCP/IP etc but very little in practice - hence my extreme confusion in why my connection is magically working considering I barely configured anything. So yeah, I do know how to configure things at least in principle, though I sometimes find this very difficult in practice due to being familiar only witha CISCO CLI and home routers have horrendous GUIs and Linux networking tools just being plain undocumented.

EDIT: I decided to add a timeline highlighting the "mysterious" aspect of this journey:

* 17th Aug: Fibre connection in the ground and ONT on the wall. Fibre plan ordered with ISP same day (btw, don't do that! wait at least 24hrs)

* 18th Aug: Phoned ISP for network settings. They told me to use PPPoE if I use my own router (but their website says to use IPoE). Both on the call and on the website I was instructed to use VLAN 10

* 24th Aug: Phoned ISP to ask why I'm still not connected and not received my router from them. They hadn't even sent it. They claimed the were waiting for Chorus to do a thing. I told them that Chorus has already done it, so not to wait. Asked them to send my router now, and focus on sorting the connection ASAP

* 26th Aug (morning): Phoned ISP to ask for tracking number for my router - they said they still hadn't sent it. But the escalated my connection and got it connected later that day. They also claimed to have requested the router to be sent immediately.

* 26th Aug (afternoon): Just after they e-mailed me telling me the connection was active, I tried plugging my PC into the ONT. It worked. I then disconnected my PC because I was concerned about security risk.

* 27th Aug (morning): Phoned ISP to ask for tracking number for router again. Found out they still had not sent it. I started flipping tables. Asked them not to send one, and went and bought my own. In the mean time, whilst waiting for my wife to come home with the router, I tried plugging my PC direct into the ONT. It did NOT work at all.

* 27th Aug (afternoon): Tried using my new router, had some trouble, ISP was unable to provide information about the settings. I accidentally fixed it. No idea. Too busy to care. Everything was happy as larry. Was using PPPoE

* 28th Aug: Found the web page of the recommended router configuration on ISP website says to use IPoE instead of PPPoE. So configured my new router to use IPoE instead. Still worked fine.

* 30th Aug: Tried using my old ADSL router (it has a WAN port), with IPoE. Everything works fine! Also tried direct PC connection to ONT, and that worked too! Phoned my ISP and found that my ONT is NOT tagging traffic with a VLAN. This (according to the ISP) is a misconfiguration.

The mystery is why direct PC connection worked sometimes and not other times, and how I can have a working connection without VLAN tagging.