Replicant

I am using Replicant 6.0.0004 on a Samsung S3 and just installed the Signal messenger app 6.1.4 using the APK download from signal.org/android/apk since I have red that some people were able to use it with Replicant.

As soon as I try to start the app it asks me for my mobile number and then informs me that it will have limited functionality without Google services. I am willing to accept that but after selecting "ok" it just tells me that it has no connection to the service and shows me the form for entering my mobile number again. But of course the mobile is connected and works fine otherwise.

I already have given the app all possible permissions and did two reboots and I have asked for help in the Signal community. But maybe someone here has any suggestions how to overcome this blockade. I fear that not many people will have relevant suggestions in other places when it comes to Replicant.