On home packages yes but not on reseller business ADSL we have, downgrading to 3.8.8_BT works fine with Teamviewer and accessing our trail app.com websites.

According to this post: https://community.teamviewer.com/t5/Knowledge-Base/Which-ports-are-used-by-TeamViewer/ta-p/4139 ,
Teamviewer uses (one of) ports 5938, 443 or 80 (with a fall-back to the next one, in that order).

This is, surely, either a firewall issue - or a DNS issue?

There were some changes to the way the 2860 dishes out DNS server addresses (can't remember the details), so this would be my first guess.

What happens if you force the allocated DNS server to be 8.8.8.8 or 9.9.9.9 for example (assuming you don't have your own internal DNS infrastructure)?

andrewepugh wrote: Plot thickens, returned the working 2860 with 3.8.9.3 back to the shops location and connected it up and same issues again. It was fine on my VDSL and WAN2 interface at my office but as soon as it got back to the shop with ADSL connection problems arose again.

Note I have BT Fibre the shop has a third party ADSL connection retailing via a Talktalk reseller... whether that has anything to do with it i don't know. Downgraded back to 3.8.8_BT and working fine.

FYI the problems I reported regarding the potential NAT problems (see the other thread in installation & Setup) with the later firmware versions only affected me when using the WAN1 connection, using a bridge modem on WAN2 was fine

I tried forcing DNS and it made no difference.

WAN1 vdsl was fine with BT - WAN1 adsl with Peach Telecom was not.

Strange thing is it is fine with similar adsl (same provider) same adsl package at a different location using another 2860 on 3.8.9.3 (so far anyway...)

Thinking it has to be this one router that doesn't like either ADSL or Peach Telecoms service.