I've got a Fiio M6 too and you can use the musicbee wifi sync
Download Apk editor and install it on a regular android phone.
Download the musicbee wifi sync apk.
Select the apk file in apk editor,
select complete editing, decode all files,
then select files and androidmanifest.xml,
once you open it change the package name by a package name of a white-listed app,
save and build the new apk,
put it on the Fiio and enjoy!