I am not a pro. I can add to RJR's post above. Disregard if desired!!
I would suggest using only TIU #1 and the fixed #1 channel if both can be used. (edit" to add engines!)
I would suggest connecting the remote directly to TIU #1 with the 4 wire hand set cord to help eliminate any issues.
If there's a club using DCS in your area maybe they could help here?
I believe there was great discussion about only adding to TIU #1 (or the lowest number TIU used in the remote) but maybe I'm wrong here?