I have a Fairphone 3 and so does my daughter. I have noticed that when she runs out of mobile data, she does not receive any sms from me. She does receive sms from other phones though even though she has run out of mobile data. As soon as her data is refilled, she starts getting sms from me again.
At the moment, there are several people (including myself) reporting bugs with FP3’s stock SMS App . Maybe you should try another one. For me, Textra was the solution.