Check device

My P50 is equipped with Sierra EM7455. You can have it seen by lsusb:

< lsusb
> Bus 001 Device 002: ID 1199:9079 Sierra Wireless, Inc. EM7455

Install Dependencies

Since I am using Debian, install via apt:

< sudo apt install networkmanager modemmanager

Now the card should appears in mmcli (as EM7455) and nmcli (as cdc-wdm0 or something similar):

< mmcli -L
>     /org/freedesktop/ModemManager1/Modem/0 [Sierra Wireless, Incorporated] Sierra Wireless EM7455 Qualcomm Snapdragon X7 LTE-A
< sudo nmcli dev | grep cdc
> cdc-wdm0         gsm         ...              ...

FCC Unlock

Since I purchased the P50 in United States, FCC regulations applies that the WWAN LTE Card has to be unlocked first.

ModemManager included the unlock script:

ln -sft /etc/ModemManager/fcc-unlock.d /usr/share/ModemManager/fcc-unlock.available.d/*

Please refer to ModemManager Documentation for more information.

Connect

Now we can create a connection configuration with nmcli:

sudo nmcli connection add type gsm ifname 'cdc-wdm0' con-name [My LTE Connection] apn [YOUR APN NAME HERE]
sudo nmcli connection up [My LTE Connection]

Now it should be OK.