I see you have issues with APN. I think it’s easiest if you contact our support team by email ( www.fairphone.com/asksupport ) or calling them (number at bottom of page: www.fairphone.com/support.
They can best help you there and feel free to write in English, Dutch, or German.
This is not the case in every situation, but we have heard from some customers who use small “MVNO” - companies that buy towers from other, bigger companies that the following happens:
Usually this roaming alert is caused by Mobile Virtual Network Operators (MVNO). These smaller phone networks (daughter network) make use of the networks of larger phone companies (mother network). This makes the phone think it is roaming on a different network although this is not the case. I hope this information helps and to make sure we do recommend asking your network provider, just in case.