Similar here (sim in my router, but have voice and text too). Was okay at first but since beginning of March, if not before, it's gone really down hill. Often pages don't load or I get the message "no internet"! But get reasonable service before 8 am.
When I pop my postcode in here...
https://www.three.co.uk/support/network_and_coverage/network_support
...it get the message...
"Weโre fixing an issue with our network in this area
Weโre sorry for any disruption you experience. Our engineers are working on masts nearby, which means the rest of our network is handling more traffic than usual...."
Have you tried that?
