No Service Notification

[MayDee]

My phone has started giving me a "No Service: Selected network (O2-UK) not available" notification. Sometimes this immediately disappears, yet other times I cannot phone or message anyone, or get access to the Internet. How do I fix this?

[MI5]

@MayDee 

Check for network issues here O2 Service Status

If it still persists, you may want to try a new sim card.

You'll need to call customer service to get one sent.
Numbers here Guide: Coronavirus Community Help and Support 
When it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.