First, the online maps are a computer generated guess. There can be many variations in a given area..
Second, signal bars on phones are only a guide and can vary make to make, model to model.
Have a look at these guides
Guide: How can I sort out my network issues?
Guide: Is the network down for me or everyone?
If indoors use WiFi Calling
https://www.o2.co.uk/connectivity/wifi-and-4g-calling
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
