Speaking of obscured ports, perhaps your ISP *is* blocking well-known ports (not likely, but at this point, I'm out of ideas). Granted, I wouldn't expect 8443 to be blocked. But maybe the ISP has a defined limited range they're willing to accept, say 10000-19999. Again, very unlikely, it's NOT the norm, but perhaps worth asking the ISP.
If all the relevant entries in the PREROUTING table of the NAT table show zero packet counts, there *has* to be a problem upstream of the ASUS router. Either the primary router is NOT forwarding packets to the ASUS for some reason, packets are never reaching the primary router for some reason, you do NOT have a public IP on the primary router, you're referencing the wrong public IP, or perhaps your outbound remote access is being blocked on the remote client device.
I can't think of much else that could be preventing the remote access.
P.S. If you want one of us to try accessing the GUI or RDP server remotely (even though we'll get locked out by username/password), at least we'll know if *we* can reach it, even if you can't for some reason. Just send a PM w/ the public IP and port (in case you changed it).