Hi kawayanan,
I was having issues with getting UPnP/DLNA working, but was able to get it to work, see link below.
Looking for DLNA for GL-MT300n-V2 - routers / software - GL.iNet
https://forum.gl-inet.com/t/looking-for-dlna-for-gl-mt300n-v2/3378/7
Hope this helps …