As a follow-up to the thread my new 2700 class DMU with its TCS decoder is now working perfectly. When received, the locomotive's TCS decoder had a long address of 2700 and would not work on DC. I reprogrammed the decoder to a short address (CV1) and it immediately ran perfectly on DCC and the TCS function only decoder in the dummy car worked the directional marker lights in that car flawlessly. When switched back to DC the DMU again ran perfectly when it would previously not do so, confirming what several members had stated, that their TCS decoders would allow the chipped loco to run on DC.
I don't have a manual for the TCS decoder but referred to my Lenz 21+ booklet. CV29 Bit 3 (which facilitates running on both DC and DCC (when enabled), versus operation on DCC only) is set by default to "1" (on), which allow DCC and DC operations.
I did not reprogram CV29, merely reset the decoder to a short address (CV1), so it appears that the CV29 Bit 3 was automatically reset to on.
It is unclear whether the CV29 Bit 3 was accidentally turned off ("0") by the previous owner disabling DC running, or whether programming an extended address alone (CV29 Bit 6) disables DC running by turning Bit 3 'off'.
I believe the TCS decoder in the dummy car is a function only decoder (without a motor control) and since both were likely programmed together as a consist disabling CV29 Bit 3 not only prevents motor operations but entirely disables operation under DC including functions (in this case, lighting)
Everything working correctly now, so happy this was such an easy fix!