If you are facing an issue (audio stop/freeze) while listening to Radio FM on your Android device, it is most likely that Power Saving Mode is enabled on your device. The Power Saving mode may disable background data, when your device goes into sleep. Due to this Radio FM will not be able to stream the online radio stations.
Please follow the video to disable the Power Saving on Samsung Galaxy S6. Similar procedure may be followed on other Android devices running Android Marshmallow 6.0 & 6.0.1, Android Lollipop 5.0 & 5.1 and Android KitKat 4.4.
For Android Lollipop and KitKat devices, you may have to go to Settings->Battery to disable Power Saving mode
Try using Radio FM after disabling the Power Saving mode.