You must open a port for Plex Remote Access to work.
The alternative is to use Plex Relay. The stream is limited to 2 Mbps (1 Mbps w/o a Plex Pass).
For info on configuring remote access, see Changed ISP - No Remote Access with AT&T BGW 320-500 - #3 by FordGuy61. Substitute your Asustor for the Western Digital My Cloud.
