Hi! That’s funny, I had some signal problems myself (but it was my fault, I was messing around in the settings).
I found that resetting network settings solved my problem.
It’s in Settings > System > Reset options
It will reset any and all setting you have for Wi-Fi, mobile and Bluetooth, though.
I did few testing.
I installed LineageOS 18.1 and it worked when I was in another place (few km from my home).
I did the same by installing Fairphone OS 21.05, and it worked when I moved to another place.
At home, it is not working for both (it is working on another phone with the same SIM card).
It seems that the FP2 antenna is not enough powerful to connect to the network compared to the Samsung one.