I've heard that holding down the horn button can determine a location where the TMCC signal is weak. If the horn stops blowing the signal is weak at that location. Forget this one , you know where the problem is.
I suppose you tried running an additional wire from the U connection on the base to an outside rail in the weak area.
Does putting your hand over the loco help?
Are both outside rail tied together?
TMCC engines must be added to the TIU that the TMCC base is plugged into if using the dcs remote.