The only changes I did to the configuration after factory reset was to disable WiFi, configure mobile as main WAN (Network>WAN) and then set APN and service mode to LTE only and hit apply - although it doesn't matter if any of these mobile settings are changed or not because after hitting apply modem does not respond to at commands any more (can be seen when monitoring the device with logread -f). I did try upgrading to latest firmware (only legacy FW is supported on my device) without any success.
Here is copy-paste from the post I made to another topic about a similar issue:
I did not quite find the solution to my problem - all I know is that starting from FW RUT9XX_R_00.06.07 to the latest FW available the communication with the modem becomes unstable. I'm using logread -f in console to monitor the device and from there it can be seen that the modem is not responding to AT commands (it seems to hang after command AT#NCM=1,1 is sent to the modem) and is unable to recover from this:
10:53:12 2021 daemon.notice netifd: ppp (18978): Connecting modem
10:53:12 2021 local1.info gsmd: gsmd send: 'AT#NCM=1,1' (11)
10:53:12 2021 local1.info gsmd: gsmd get: 'OK' (2)
10 10:53:12 2021 daemon.notice netifd: ppp (18978): OK
10 10:53:18 2021 user.notice NCM.SH: ppp_4: Connection state: Timeout.
10 10:53:24 2021 user.notice NCM.SH: ppp_4: selected IP:
10 10:53:30 2021 daemon.notice netifd: ppp (18978): Timeout.
From the GUI (luci) under Status>System when temperature is shown as 'N/A' or on the overview page you see Timeout. being displayed then this is also a tell tale sign that the modem is not responding to AT commands. With firmwares starting from 00.06.07 I get that a lot. From monitoring the device logs I can see there is some modem_check script which is sometimes able to reset the modem but it doesn't seem to be running all the time.
I checked the ncm.sh scripts between releases 00.06.07 and 00.06.08.6 and they are identical. In release 00.06.06.1 the script is slightly different. Coincidence? Could it be that the newer firmwares are not properly tested on legacy devices which are using a different modem?