Registration and use of the iPhone in the same network is possible without any problems. ROON Core says ARC can access my server.The APK APP has already been reinstalled. Tried APK APP from the PlayStore and from the ROON download page, the result is the same for both.
All clear, the solution came with the hint of the browser settings (point 1). I thought which browser it wasn’t installed. Play Store etc. worked, I didn’t need anything else. There is no browser installed out of the box, I installed Chrome and it works.