Hi,
Using OTG is shady as each Model can have customized implementation by Manufacturer.

Problem is that OTG drives are not correctly mounted and reported as Removable Storage whose ID is not constant which result in Grayed storage (eg. ID has changed on new connection/restart/return from sleep). When you play track in MMA, but initiated from Samsung File manager, can you check File properties and see the path of File (I suspect it will have some strange beginning and a Screenshot could be good, along with Debug log [Write ID here before sending]).

One thing I am curious, have you tried to use MMA Folder browser and does it show connected OTG drive?

Has anyone had any success with using Media Monkey on Samsung Tablets. I attached an external hard drive to the tablet, but Media Monkey will see the library, but it will not let me play anything from the with in the library, I have to go the tablet file manager click on the file and then it will give me the option to use Media Monkey, that only works accessing one file at a time. When I click on ad Library, I can see the library but it is greyed out.