From all I know, Signal now requires that your device runs at least Android 4.4. If you are still on the Fairphone OS (Android 4.2.2) that your FP1 came with, you can upgrade it yourself to z3ntu’s unofficial KitKat (Android 4.4.4) for FP1:
In the process of this upgrade, you probably need to completely delete all data on your FP1, so if you need to backup anything, you need to do it beforehand.