Firstly check in your phones network settings to ensure you haven’t accidentally disabled 4g (it happens I’ve done it🙄) Then check the O2 network app to check for issues at your local mast. Can you get 4g via another mast? If so the fault is with your local mast and the O2 network app should if you log a fault keep you updated.
Have you got another phone to put your sim in to see whether 4g works on that?
It’s a process of illimination.
A few things for you to try.
Good Luck 🤞