Just back to N.I. from a sort weekend in the ROI found that my phone cannot connect to any APN, appearing all as forbidden. Try with a spare old phone and I´m able to connect to a T-mobile UK network and it works.
Tried my main phone with a Spanish sim card when I´m abroad and it´s accepted and work. APN are configured and connect to a O2 network (roaming config)
Only difference is the VM one looks very used in the meantime the Spanish one looks new.
Guess if is possible the issue comes from the sim card.
Main phone - XiaoMi 3
Spare phone - Samsung Galaxy S1
It´s a bit odd be carrying on both phones in order to be available.