In the DJI Go App, choose the MENU option on the right side of the screen for Images and Video. Choose the Video icon and change file type from .MOV the default to .MP4. Choose the Option tool icon and TURN ON Video Caption. That option needs to be ON for the .SRT files to be saved with the .MP4 on your SD Card when you record during a flight.
Please note: you must have a DJI model that properly logs the GPS as an SRT file.