IMO this whole thread is not about a Fairphone bug but about an Android 5 bug. I think I did not have it on my former Android 4 phone.
- phone network selection always behaves as automatic, therefore it is impossible to stick to chosen network when chosen network weakens; Phone will automatically switch to a stronger signal thus leading to roaming
- PLMN management does not work : impossible to save a new PLMN and equally impossible to erase an existing PLMN.
The problem should be addressed by Google for all Android 5 phones.
I have not been able to find a solution yet.