The TRGPro was built on version 3.0 or so of PalmOS. If you have a Linux box, the standard utilities like jpilot and pilot-link should work with it. If you're on Windoze, look for a Palm Desktop CD from a Palm III or Palm V.
The PalmOrb app (it's still on sourceforge) will let you use it as a 4x20 serial LCD with a Matrix Orbital command set. There were also freeware terminal programs for it (one of them was called "somethingtelnet" or "telnetsomething", as I recall). Unfortunately, most of the old Palm software sites have either died, or turned into repositories for cellphone software. But there are still a few out there.
I don't remember the "wireless" option. Was that a CF card with WiFi?