They do use also port 443, but that is initated from within your local Plex server and doesn't need a forwarding rule in your router.
I have just set port 32400 fixed in Plex and in my Asus and it is working perfectly. Have also a look at your firewall rules at your Plex server itself.
Did you tried the rescue mode of the Asus?
https://www.asus.com/support/FAQ/1000814/
It happened to me and I nearly every time got it back to life (also with bricked routers from other people)