Requests to Internet fail temporarily on Device attached to RAX80 since recent update

We believe since we have updated the firmware to Version V1.0.4.120_1.0.61 (or the update before that) we have had some stability issues.

I can confirm it is a router issue as we have a second one which is independent (both connect to our ISP separately) and has no such issues.

These are the symptoms:

1. Some mobile phones (not all it seems) connected to the router will complain there is no internet available
2. On a wired connection to the router from chrome/edge it will complain it cant load the page with some sort of not found error... the browser will self retry and then load the page successfully.
   • Note that this also occaisonally causes stylesheets not to load... 

This router was not cheap and is less than 12 months old (just out of support)... getting a bit frustrated.