Certain sites/apps not connecting

I recently had a tech upgrade our home from T3200M and wifi boosters to NH20A with wifi 6 boosters. Browsing seems generally ok although a couple of drops however the main issue is around apps and sites that require a login. These show as an error message like “We cant seem to find an internet connection” while I am browsing just fine on the same network at the same time.
I have tried the following fault finding:
Different browsers - fault is still present
Different devices (Windows versus Mac, cell, tablet etc.) - fault is still present
Hardwire versus wifi - fault is still present
Turning off wifi and accessing via cellular - issue is resolved.

I even have to be on cellular connection to be able to post in here as it requires a login!

I checked the firewall settings on the NH20A and it’s showing as ‘High’ and nothing ticked in the ‘Traffic In’ column.

Engineer was meant to be here yesterday but was a no-show.

Thoughts?