Right...happy to provide an answer to my own question...
Seems that this problem is pretty common, and there's just one small but obvious (in hindsight) solution to this problem. I bought an unlocked phone that was originally set up for AT&T. But I use it with my Telstra SIM card. And like a knucklehead, when I went to edit the ip_modem_configuration.xml file, I went to the telstra section instead of the AT&T section. The Telstra section already had enabled="true". But the AT&T section had enabled="false". Change the AT&T setting to enabled="true" and the "Mobile Internet" option has come back.
Hope this helps someone else...