Starting Marshmallow 6.0, Android launched a feature of Android Doze. Android puts the device into Doze mode when it is not in use for long period of times. When the device goes into the Doze mode, the Network access is restricted due to which Radio FM may not be able to access the Network and hence not able to stream the online radio stations.
For a seamless experience, you need to remove Radio FM from the “Optimise battery usage” settings. Please follow the video to check how to do that