By any chance, are you using a modem / router provided by your ISP? I have noticed that with some of the HITRON cable modems, you can end up in a situation where you are "double NAT'ED".

On my setup, I have Ports 2456-2458 TCP and UDP forwarded to a specific IP (the machine running the dedicated server - which is distinct from my gaming PC).

Try this tool -> https://www.ipfingerprints.com/portscan.php - select Advanced - Ping Type "
Don't Ping" - General Options- UDP Scan.

You should see output similar to mine:
Host is up.

PORT STATE SERVICE

2456/tcp filtered altav-remmgt

2457/tcp filtered rapido-ip

2458/tcp filtered griffin

2456/udp open|filtered altav-remmgt

2457/udp open|filtered rapido-ip

2458/udp open|filtered griffin

If you don't see this, and you have a modem provided by your ISP AND a separate router, I suspect you are double NAT'ed.

I'd confirm that settings like "Residential Gateway Function" is set to enabled, consider enabling uPNP which will allow the other router to open the port on the ISP modem.

Hopefully some of this stuff helps - I know that with older games like UT3 for example, the type of NAT that I had "symmetrical" vs. "full-cone" made a difference, but I hope it's not the case here.

NOTE - none of this is based on using the same PC that you game on as the dedicated server, I would refer to guidance elsewhere for that - my setup is based on a dedicated machine running the game. If you want to try one more option, consider running it in a VM to see if this helps, because to me it's still sounds like NAT or Firewall.

Hope this helps!