I was having an issue with the same symptoms - great signal but no data / nothing loading. It was happening nearly all the time on my iPhone 12 pro on o2-uk, pretty much any location I went to. found myself constantly rebooting phone, or going into airplane mode, waiting a few seconds then going out of airplane mode normally fixed. o2 adamant it was phone issue and nothing wrong with their network or my account. Tried all releases of iOS 14 since the 12 launch, beta /non-beta, resettings network settings, changing APN details, then going to iOS 15 - which also didn't fix. I was incredibly frustrated and could only assume my iPhone 12 had a faulty modem...
So I eventually replaced phone for iPhone 13 mini on iOS 15 when it came out, expecting it to fix the issue BUT NO! still had the issue - still on o2. eventually o2 let me leave my sim only contract, I went to VOXI / vodafone and issue disappears. Defiantly something to do with how some networks have configured something in some areas.
When I look on twitter there are defiantly people with exact same issue with me, mostly in the US, but also many people on o2 in the UK in certain regions, and now I see this thread also.
I never got to the bottom of it - seems like some very specific carrier configuration that upsets the x55 / x60 modems in the iPhone 12 / 13. Perhaps something to do with the carrier aggregation / mix of TDD and FDD, or related to switching from mast to mast so many times, not really sure what triggered it. Try a SIM on another network, I bet it fixes it.