This might help others. I recently took out a SIM only contract with BT having previously been with another UK provider. SIM went into iPhone 8 and all worked well in UK. Came on holiday to Europe. Did all the right settings before leaving but only voice and SMS worked. No data service. BT lead me through all their suggestions. Reset network setting on phone. Switched off phone and take out SIM and restart. Manually set roaming network. Nothing worked. Then I found issue. The previous provider had a profile still present on the phone. Deleted it (General / Profile) and all was good. Hope helps someone.