I've had this problem for a while now, I have a dual SIM OnePlus 8 Pro. The O2 SIM is my main SIM and Vodafone my 2nd sim and only used for calls and texts to that number. The trouble I'm having is when outside the home and not connected to WiFi I can have full 4G signal but no internet. Had the same problem with old phone a Xioami Note 10 but not as bad. I've taken the Sims out, restarted turned off, factory reset and still keeps happening. I can't think of anything but new SIM card needed?
Has the internet never worked on the o2 sim card or has it stopped working @Orang3robot ?
Check the APN settings (link below) and make sure they are correct:
If everything is ok, then yes trying a new sim is an option.
There are some alternative numbers to call, as well as hints on getting through, in the guide below:
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
Track usage, upgrade and get support with the My O2 appDownload My O2