@Kevin_Z Hi thanks

So I have reset my network once more and used the TD-W9970 as a router/modem on factory settings to find this setting.  It seems I didnt test enough last night as when using the factory automated settings my pings as stable.  I have tested this for 2 hours and its remained stable at 10ms ping constantly.  The setting I was on about was called Echo Request Interval which was called PPP Echo on my older Asus modem/router (I guessed these were the same thing as the number variances matched).  I thought that was what was the fix but it turns out it wasn't, as on the default 30 my internet was perfectly stable and fine.  So i apologise for that miss information.

After 2 hours of having this stable, maxing out my bandwidth and seeing nothing but a solid 11ms ping, I decided to try the C5400X router once more.  I factory reset the TD-W9970 again, put it into bridge mode, VLAN 101 again and connected everything up.  The C5400X instantly connects to the internet via this, but once more my pings are unstable and have remained so for an hour.

Here is a little snippet of what it always looks like:

Reply from 151.101.0.81: bytes=32 time=10ms TTL=56
Reply from 151.101.0.81: bytes=32 time=9ms TTL=56
Reply from 151.101.0.81: bytes=32 time=15ms TTL=56
Reply from 151.101.0.81: bytes=32 time=11ms TTL=56
Reply from 151.101.0.81: bytes=32 time=10ms TTL=56
Reply from 151.101.0.81: bytes=32 time=23ms TTL=56
Reply from 151.101.0.81: bytes=32 time=13ms TTL=56
Reply from 151.101.0.81: bytes=32 time=13ms TTL=56
Reply from 151.101.0.81: bytes=32 time=14ms TTL=56
Reply from 151.101.0.81: bytes=32 time=10ms TTL=56
Reply from 151.101.0.81: bytes=32 time=17ms TTL=56
Reply from 151.101.0.81: bytes=32 time=11ms TTL=56
Reply from 151.101.0.81: bytes=32 time=13ms TTL=56

Again, without the router being used, both my TD-W9970 and older modem router shows time=10ms constantly.  Ultimately I wouldnt want to use the the C5400X as an access point due to me needing the port forwards, I wouldnt want to be behind double NAT.  I could always test it to see if the pings are resolved doing this, at least it could help narrow down the issue.

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Whilst typing this out I thought i'd click around in the router settings. I decided to turn off the Antivirus and Firewall whilst my ping tests were running, once all these were turned off my results immediately became this:

Reply from 151.101.0.81: bytes=32 time=9ms TTL=58
Reply from 151.101.0.81: bytes=32 time=17ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=9ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=12ms TTL=58

Reply from 151.101.0.81: bytes=32 time=14ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58

Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=9ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=9ms TTL=58
Reply from 151.101.0.81: bytes=32 time=9ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58
Reply from 151.101.0.81: bytes=32 time=10ms TTL=58

Turning the protection back on didnt seem to hinder the pings again.  It still has the odd ping fluctuation which I dont get without thethe C5400X but it appreas for now the pings are generally far better then they were.  I will leave this a few days and comment back here with any results good or bad. Could this had been a software issue within the the C5400X? :)