Hi there, I have been facing the same issue after upgrading my iPhone 8 to iOS 14.4. The mobile network is either very low or mostly it shows no service. The network shows up once or twice in a day but still I’m unable to receive or make calls nor receive any text messages. Whenever I place a call, the available network turns to no service. Tried consulting the apple support on this but they aren’t aware how to resolve this. They would just simply tell me to try rebooting or reseting the phone. This issue of fluctuating 4G signals started around 6 months back for me when I had upgraded my iPhone 8 to iOS 14. Until the past two weeks the 4G was fluctuating but after installing 14.4 its even worse and shows as no service.