The UI is being reworked right now to better fit the screen, but it's generally there in GNOME Settings (latest changes broke the password field and I think it's not restored yet, but you can fill it in by editing NetworkManager's config anyway). For LTE you just have to set the correct APN. I've just connected to the internet with LTE to test it and it worked; and I'm using WiFi connection daily.

Make sure your kill switches actually turn WiFi and LTE on. Also, the modem is generally way more stable when used with battery inserted, as USB might not be able to supply enough current.

Turning on power switches to the chips is pretty important!

It gives a hint about how manufacturing is, that the switch tops come out so late.