Let me see if I understand what you've just said. You can't add an engine to a remote when it's connected to one TIU but you can when it's connected to a different remote, and then the loco works with either TIU? If I understand correctly your situation, to me that means that there is something wrong with the first TIU, and the remote is fine. Have you tried the remote tethered to the first TIU to see if it will add a loco then?
With this recalcitrant TIU, if you with the remote access TIU setup and then TIU version, what software version does it say? If not 5.0, it didn't upgrade.
One other thought; do you know what version of software had been in this TIU? Reason I ask is that if it were version 2.#, after upgrading to 3.# or higher, the TIU has to be reset to clear out data that had been stored in the TIU under version 2.# but not in later versions.
