My SIM card is locked with a PIN and I noticed during the multiple reboots that when the phone first boots up to the SIM unlock screen up it was showing a phone signal, and when I entered the PIN the phone signal disappeared briefly and then came back. I figured this might be the point that 4G is lost, so after I managed to get mobile data working again after a reboot I was able to disable the SIM lock (when I lost mobile data the SIM lock option in security settings was greyed out). Since I've disabled the SIM lock I've booted multiple times without issue. When I re-enabled the SIM lock the problem came back straight away.
Can anyone suggest what the problem might be, and how I can re-enable my SIM lock without losing 3/4G connectivity after most boots.