How to stop FaceTime and iMessage alert

I have a data only sim which has poor signal at home. Every time my phone loses signal and gets signal again it displays this message.

Möglicherweise fallen vom Netzbetreiber SMS-Gebühren beim Aktivieren von FaceTime und iMessage an.

which translates to

Your operator may incur SMS charges when you enable FaceTime and iMessage.

If I leave my phone for a long time it connects to the network maybe 30 times then I have 30 alerts to dismiss before I can use my phone.

I really don’t want to change my number as I’m only using a data only sim while in another country.

Whenever you lose your cell connection, and reconnect again, Facetime / iMessage may need to verify if your cellular network has changed and thus the SMS request pop-ups. (Nobody at Apple must have thought of your scenario of frequent disconnection as that is rare).

See this answer for a short term solution - and contact Apple support and report your issue so that they can hopefully fix it.