@Gastomagic 

Turn off SIP ALG in ALG settings in securoity tab.

There is also another reason this happens, wifi interference, is the microwave in use when it happens?

 ok I've done that. It doesn't explain why the issue has only just occured after two months of use though

  @Gastomagic 

Can't say for sure I just know SIP can mess with calls & confrencing.

Is it working ok now?

  @Bogle no the issue remains. The router is behaving very strangely. Different devices randomly disconnecting. Also the upload speed seems randomly far higher than download when connected. 100mbps DL Vs 900mbps UL. I plugged my old router back in and issues disappeared and UL and DL are about the same values. Also when viewing things like Facebook when I scroll down it stops at each page as if it isn't downloading the next content in advance. This disappears when using cellular data or the old router 

  @Bogle 

Also I've run some ping tests.

The tplink router after a reboot behaves fine for 30 mins or so - then has problems. 

I plugged in my old retired router to provide a comparison.

Tplink result 

Then result from old router

As you can see from the comparison - the tplink shows a shuttering behaviour. 

  @Gastomagic 

Yeah you deffo have a lag there but why is one on one domain as such and the other on another, cloud vs jt?

I think that's going to be the problem but why is it doing it in the first place surely they should both report the same especially as they're both on the same ip address as well.

Have you changed the DNS at all? if you have try another one,if you haven't and they're isp servers try Google 8.8.8.8 or Cloudflare 1.0.0.1 or Quad 9 even maybe start out with experimenting with all variables of DNS and see if any are better than the other.

  @Gastomagic 

Easy one to miss but it's 8.8.4.4 for Googles back up DNS servers.

  @Bogle good point. Just a typo though. I have it set as 8.8.4.4.