Do you have a USB drive attached to the RT-BE92U?
If you haven't done so already, check if the USB Application > Server Center > Media Server feature is enabled in the router GUI. Disable it, if it is enabled. If you do not have a USB drive attached then you may want to check all the options under USB Application > Server Center to ensure they're disabled.
PS: If you are running any addon scripts, you may want to check if any of those scripts either use minidlna or use port 8200. Also check if you have enabled port forwarding on port 8200, or have configured any other router services to use port 8200 (like VPN). Don't know if UPnP would also play a roll but might want to look at disabling it too on the router, if it isn't already disabled, as a troubleshooting step.