Sorry to hear of the signal/service issues experienced with your mobile since moving to O2, we appreciate you taking the time to raise this via the forums.
Invariably signal issues aren't a result of the handset itself but more the service in the area, via the handset you can ensure you have it set to 5G. This can be done via the Settings - Connections - Mobile Networks and making sure network mode is set to 5G.
It is more likely to be a problem with the service in the area, you can check this here and can check local masts for issues.
O2 should give the same reception as O2
If you follow the like supplied by @Robert_P use the Status Tab and not Coverage. The latter is not real time and does not take into account faults
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Have had the same problems where I live.
Swapped to O2, signal has gone through the floor. Despite the O2 status claiming everything is fine, both 4G and 3G, I am now unable to get a decent signal in more places than I can.
'switch to 5G is the advice' Fine if O2 actually provided a 5G signal in my area. (sorry, 5G is currently not available in your area)
This is the reason I am off to '3' who have full 4G AND 5G that actually WORKS in my area, and it works everywhere, unlike VM/O2!