I don't know so much about Ampache but you have to use something like
http://yourdomain.XXX:8080 or
http://yourdomain.XXX:8080/Ampache for your server settings inside the app. Just the same how you connect to the interface
Subsonic App is from 2018, Ampache update is just a few days old, may be it will not work with newer versions.
I suggest using DSUB look here
https://github.com/daneren2005/Subsonic/releases it is just updated. For installing the APK from Website, you have to switch your phone to developer mode.
In the past Ampache have had a couple of issues using the subsonic.api, the reason why I didn't try it. I don't know if they are fixed.