Do you have many, many small files to scan? USB drives tend to be slow for this type of access and OpenWRT may not be as optimized as RPi. The GL-A1300 and other GL.iNet routers have built-in DLNA that you can use instead of Samba to your media player.
For media sharing on my internal network, I personally just allow Anonymous Access (guest), so credentials are not required for access.
The “no device detected” may be a glitch. You can try rebooting the router to see if it goes away.
I do not work for and I am not directly associated with GL.iNet