Strange problem (routing?)

On Plusnet ADSL service. Have been trying for a while to work out what is going on, certain sites don't work from android phone over WiFi, but by either using mobile data, or a VPN over the WiFi they work. Accessing via desktop always seems to work.
Examples are Ebay App, it only partially works over Plusnet, but taking the other actions it works fine, Then last night I followed a link to thetimes.co.uk, and it suffered the same problem as ebay app.
I'm at a bit of a loss as to how to narrow down what is happening.

I wonder if it is because the mobile is using different DNS servers to the desktop. Possible that the DNS the phone is using is returning an IP that for some reason isn't routing over PlusNet but that the PC is using PlusNet's DNS servers and therefore gets another IP. See if you can check what DNS servers your phone is using and compare to the PC.

They are both using Google 8.8.8.8 & 8.8.4.4

OK, then there must be something in the Android software but afraid I don't have an Android phone so run out of options.

It sounds like it could be an MTU issue... If you visit this web-site from the android phone or your PC, what is the MTU and MSS values?
https://www.speedguide.net/analyzer.php

Just checked MTU from phone is 1470, via WiFi or Mobile data, PC is 1492.

But think I may have found what's causing it to go wrong. Things always used to work, and suspect its changes to the App somehow. I have a TL-R470T+ in line to allow the option of balancing over mobile data, and also its options for QOS are quite good, handy on a slow ADSL line. If I connect direct to plusnet router the phone works, so looks like something the App is doing is getting upset by double nat, strange when PS4 can cope with that fine.

It's possible the app is making multiple calls but that the servers have affinity (ie all web calls should end up at the same server). If they are taking 2 different paths they might be ending up at 2 different servers and therefore session affinity is broken.

The VPN would resolve this as it forces all calls down the same connection.

PS - you didn't tell us you were balancing 2 connections you only mentioned PlusNet...

Edited by ian72 (Mon 25-Nov-19 11:06:52)

The problem was happening on plusnet, the mobile side is only on occasionally. So only the one route, but when using a VPN would eliminate the double NAT