I've seen other people ask the same thing another forums. It's not just your device, their phones were flagship models so it's more like the App's issue... OR the network connection is just not strong enough to send/receive live feed. SInce you say it's okay when using WiFi, the RING camera should be sending the signals alright... hence it's the phone's network strength that's likely the culprit.
Try looking around for stronger signal areas and see if that helps (signals are strongest when there's not a lot of metal/cement obstructions and near a cell tower). If, even after that, the live feed is having issues... then either check with the RING company directly or submit an issue report to Samsung Member app right after you see that the feed isn't working.