If there is an error during stream playback check the error message and make sure the URL is still valid and the phone is in the same network as the FritzBox device.
If the channels changed, maybe you need to run channel scan on your FritzBox again and reinstall the app. Please open settings, decoder and try VLC and ExoPlayer. If both are not working, please contact me.
If the stream is not played back smoothly this can be due to several factors. The app runs absolutely stable and without dropouts on most devices. In most cases the interference is in the environment. The fact that other apps like Netflix, Prime Video etc. work without problems does not say much, because these apps have the possibility to adjust the stream on the server side and to compensate for disturbances. This app must play a constant stream over the network and is therefore susceptible to interference.
Please check the following points: