3G modem
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ZTE MF591 (T-Mobile) -
Novatel MC760 (Virgin) -
Novatel MC760 (Ting)
According to Darren the following list is also supported but untested: link
In the web interface click the 3G page then look at the connection script. You'll find it runs lsusb and pipes the output to awk to find product and vendor ID pairs. This is followed by a switch case. You'll notice that if the PID/VID equal that of, say, the Ting modem it'll use uci to set network config options, then usbmodeswitch to “eject” the cdrom, then rmmod and insmod to add the modem to the system, usually they come up as /dev/ttyUSB0. Finally some iptables mojo is done for routing and the rest is done automatically by pppd.
To add a new modem, copy and paste one of the switch cases and replace VID/PID with your values, adjust the modeswitch command, rmmod, insmod, and you should be good to go. For CDMA there isn't really any uci network config stuff to change, the defaults should work. For GSM you'll need to specify the apn and depending on your modem you may also have to specify a pin. I haven't run into this personally, I think it's just for locked devices.