4G LTE Modem setup on Ubuntu Server 18.04

Ubuntu Server 18.04 uses netplan instead of NetworkManager by default. ModemManager is still required to deal with modems and it requires NetworkManager to be in charge. Switch the renderer in /etc/netplan/01-netcfg.yaml

network:
  version: 2
  renderer: NetworkManager

You can still define ethernet interfaces, etc in the netplan as normal. Run netplan generate and netplan apply after changes.

apt install modem-manager

Look for interface name. In my case, it wasn't the one shown by ifconfig. It was the one shown in mmcli -i 0 and mmcli -i 0. In my case, cdc-wdm4

Add the NetworkManager config for it as follows:

nmcli c add type gsm ifname cdc-wdm4 con-name NameOfYourChoice apn whatever.example.com

After I unplugged assorted USB devices (keyboard, mouse, etc) and rebooted, the usb devices got renumbered (e.g. cdc-wdm0) so the network failed to come up. It turns out you can go into the generated config file /etc/NetworkManager/system-connections/NameOfYourChoice and comment out the line defining the interface-name. At that point, it seems to find the correct device regardless of device name assignment at boot.