EM7421 multiple AT command chanel?

Establishing data channel using PPP is not recommended.

You can change to use MBIM in AT!USBCOMP.

For example, below are some steps to use MBIM in Ubuntu 18:

  1. sudo apt-get install libmbim-utils
  2. Modify /etc/mbim-network.conf to include APN/username/password of SIM card
  3. sudo mbim-network /dev/cdc-wdm0 start
  4. sudo dhclient wwan0
  5. “ifconfig wwan0” should show IP address
  6. ping 8.8.8.8 -I wwan0

Below is the log for your reference:

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: '454003074155184' SIM ICCID: '8985200012741551847' 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)

You can also see here for other method to establish data channel: