Yesterday my Fedora 23 did not show up the 3G-Modem (USB) in Network Manager Mobile Connections (using KDE; Kerndel 4.3). Having a look with dmesg reported a registered modem…
Solution was easy – sometimes Fedora-Installations don’t start and enable ModemManager. So with starting and enabling it, everything worked fine:
systemctl start ModemManager.service
systemctl enable ModemManager.service