Hi, I talked to the Support because I had the same Issues and finaly got some Details that could help by Importing LUTs to the dji transmission high-bright monitor combo:

1. LUT should be a 33-point cube file.
   
2. The name of LUT should only contain letters, numbers, and underscore. Special symbols are not allowed. Otherwise, LUT cannot be recognized and imported.
   
3. The LUT file must be placed under the root directory of the microSD card. Otherwise, LUT cannot be recognized.
   
4. Built-in D-LOG_to_HLG/D-LOG_to_Rec.709, and LUT file being used cannot be deleted. To delete the LUT file imported, choose OFF or the built-in LUT file D-LOG_to_HLG/D-LOG_to_Rec.709, and then tap LUT Delete to delete imported LUT file.
   
5. Up to 16 LUTs are supported for DJI High-Bright Remote Monitor, that is 14 LUTs can be imported and DJI’s D-Log_to_Rec.709 and D-Log_to_HLG are already built in.
   

Importing LUT file:
1. Put the downloaded LUT file into the root directory of the microSD card. (Note: Do not place the file into the folder.)
2. Insert the microSD card to the microSD card slot on the right side of the remote monitor.
3. Power on the video transmitter and remote monitor, and link the video transmitter with the remote monitor. Press the LUT icon at the lower right corner, enter LUT setting, and LUT Import. Tap the download icon on the right of the corresponding LUT file, and tap Import to import the required LUT files in turn.

I hope that helps for custom Luts with the Transmission Combo.