Remote access configuration options if your router does not support “UPnP” or “NAT-PMP”?

Plex Media Server Remote Access
2

Use manual port forwarding.

See Troubleshooting Remote Access support document for details.

Run PMS 1.19.2.2737 or later. Earlier 1.19.2 builds had an issue where PMS would incorrectly indicate remote access was not working.

This post shows port forwarding statement in an Asus router.