Just keep rebooting your phone until the sim comes online.
It can take up to 24 hours for service to be fully restored.
You can't just switch numbers as the first number ported will cancel that sim.
If you want to get that number back, your only chance is to call customer service to do it.
Numbers here Guide: Coronavirus Community Help and Support
Or advice from an O2 Guru regarding a way to get through:
If you call 202, 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.
Track usage, upgrade and get support with the My O2 appDownload My O2