I don’t know how to make your PC stable.
Maybe you can try with Windows and see if problem only exists in this ubuntu PC.
For NR signal, you need to generate some traffic like ping google operation in order to let module register to ENDC mode in AT!GSTATUS?. Otherwise, it would stay in LTE mode.
Here is an example to set up the MBIM interface for data establishment:
owner@CNHKG-EX-001367:~/legato/workspace/git/mangOH_2-7-2019$ cat /etc/mbim-network.conf
APN=hkcsl
APN_USER=
APN_PASS=
APN_AUTH=
PROXY=yes
owner@CNHKG-EX-001367:~/legato/workspace/git/mangOH_2-7-2019$sudo mbim-network /dev/cdc-wdm0 start
Loading profile...
APN: hkcsl
Querying subscriber ready status 'mbimcli -d /dev/cdc-wdm0 --query-subscriber-ready-status --no-close'...
[/dev/cdc-wdm0] Subscriber ready status retrieved: Ready state: 'initialized' Subscriber ID: '454003074123456' SIM ICCID: '8985200012741123456' Ready info: 'unknown' Telephone numbers: (0) 'unknown' [/dev/cdc-wdm0] Session not closed: TRID: '3'
Saving state... (TRID: 3)
Querying registration state 'mbimcli -d /dev/cdc-wdm0 --query-registration-state --no-open=3 --no-close'...
[/dev/cdc-wdm0] Registration status: Network error: 'unknown' Register state: 'home' Register mode: 'automatic' Available data classes: 'lte' Current cellular class: 'gsm' Provider ID: '45400' Provider name: '1O1O' Roaming text: 'unknown' Registration flags: 'unknown' [/dev/cdc-wdm0] Session not closed: TRID: '4'
Saving state... (TRID: 4)
Attaching to packet service with 'mbimcli -d /dev/cdc-wdm0 --attach-packet-service --no-open=4 --no-close'...
Saving state... (TRID: 5)
Starting network with 'mbimcli -d /dev/cdc-wdm0 --connect=hkcsl --no-open=5 --no-close'...
Network started successfully
Saving state... (TRID: 6)