I have been having similar issue with my iPhone 11 everytime I leave a wifi area I can’t get mobile cellular data unless I reboot the iPhone. It started several months into my purchase intermittently but now is happening all the time. Very annoying as I have been to Apple support and despite several visits and hours of trouble shooting still no resolution. I have swapped my SIM card and asked O2 my carrier to checking settings. It seems it’s either an iPhone 11 issue or a subsequent iOS update issue as it interacts with this particular model. I am tempted to insist for a change of model iPhone as no satisfactory solution is forthcoming