Asking out of curiosity: which version of Signal are you running?
On my XA2, Signal runs just fine on 4.5.0.16 without having microG installed at all. (My current Signal version is 6.7.6; I always download the APK directly from their web site whenever I feel like it or when Signal starts nagging.)
Ensure you have Fake Store installed in version 0.1.0 (or later, I guess). If you haven’t installed it before, get F-Droid and install it via F-Droid. If you installed it before using F-Droid, ensure it’s at 0.1.0+ as described in the linked post. If you installed FakeStore by any other means, follow the same procedure to install 0.1.0+.