What happens when you do? It isn't shown in the NetworkManager? My Solus has "Gnome modem manager" preinstalled which appears to be for working with 3g modems in the Networkmanager. I'm not sure if it's the same, more or less, as https://snapcraft.io/modem-manager and its companion network-manager. As for "Modem Manager GUI" it's not in the repo, it doesn't seem to be a snap or flatpak and I don't see a github repo with building instructions so I am not able to assist in that endeavour.