Is the device in an area of poor signal?
In particular, is it in an area with poor signal on the SIM’s “home” network, and (much) better signal on other networks - but with no roaming rights to those networks?
In that situation, the device is prone to lose the “home” network (due to poor signal) and camp on to an “other” network in “Emergency Calls Only” mode - and get “stuck” there.
AFAIK, the only escape from this situation is a reboot.