i get this issue constantly as well. about half the time, the cause is that i have two default IPv6 routes. removing the incorrect gateway fixes it. the other half of the time, i have no idea what causes it, but i have no mobile network
disabling VoLTE and restarting ofono fully breaks my mobile network stack, tho, and i must fully reboot to get mobile data working again.
disabling VoLTE and NOT restarting ofono fixes it sporadically.
also, i cannot just remove IPv6 routing as my telecom (T-Mobile USA) requires IPv6 for MMS.