Internet Issues - SLINGSHOT

Hello all - My family and I are at our wits end with an internet issue

We have been dealing with Slingshot and she's been a hard slog

My wife, computer illiterate and my 13 year old knows more than me

My wife has talked a few times to the customer service people and I have both talked and emailed the tech support team

In a nutshell, there is a download issue, it crawls and only barely, you connect fine on either wireless or ethernet but any download does not function

Slingshot have a process that they like you to follow, involves DNS Server changes, Running a script (bat file) etc, we have done all sorts, Slingshots requests and our own investigations, have tried a lot of things

Tracert and pathping using the CMD function shows the culprit, early time out either at or near modem

We have had very little in the way of direction of basic help from SS

SUMMARY

Line Speeds are fine - both wired and wireless, no issues - tested plenty

New Modem has been reset plenty

Affects PC and Laptop to the same level

Tried both PC and Laptop at another location under a different network and all fine .... so dont believe it is any settings or firewalls in any computer

We can read Stuff and Trademe and vaguely watch youtube with plenty of buffering going on, when you download anything it will take 2 to 15 hours

Any ideas on what else we can try ?

Much appreciated - Cheers

Aaron and Jenny