@Strugman It most likely had to do with the time of night you were attempting to remove it. O2 updates their systems overnight around that time. If you try again this morning you should be able to remove the bolt-on.
If you're unsuccessful, below is the advice from an O2 Guru on how to get through to customer service.
Call 202 from your mobile.
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.
Other numbers are available here: Guide: Coronavirus Community Help and Support Because you're not buying anything, O2 doesn't consider your call important, so use the Lost/Stolen or Fraud option if you have to, to get through. Otherwise you'll just be disconnected.