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:
You can cast streams using Google Cast within the app to a Chromecast compatible device. As the stream is not compatible with Chromecast, it needs to be transcoded on the fly on your phone/tablet. Therefore this only works for HD channels because it uses hardware accelaration.
Instead of Chromecast, it is recommended to use a Android TV / Google TV (e.g. Google Chromecast with Google TV) or Fire TV device and use my native apps there. It is still possible to send streams from your phone/tablet, but in this case, the decoding is done on TV without quality loss.