Did you try to enable the beta VoLTE? It seemed to do nothing to me yesterday when I tried it, but today woke up to phone stuck on 3G and no data connection, disabling/airplane mode and reboots (though it would flash 4G for a second after reboot it would drop back to 3G or even go roaming on another operator soon after) wouldn’t help, only the trick #3 you mention helped (at least for now), keeping VoLTE disabled and hoping it was the culprit as the phone will be unusable without data