In hope of reconnecting my Honeywell T10 thermostat, I downloaded the trial version of HomeKitHub professional. It appeared to download, but showed as a big red "disconnected" and running discovery did not find the thermostat.

Did you do this as mentioned on the announcement post? Please make sure you have done a recent Upgrade Packages: https://github.com/jimboca/udi-poly-homekit-hub/blob/main/DEBUGGING.md#common-causes

If so, review: https://github.com/jimboca/udi-poly-homekit-hub/blob/main/DEBUGGING.md#discover-does-nothing-or-adds-no-rows