Ok so I just went to Slate AX (GL GUI → VPN → VPN Dashboard → and click on the configuration file I see the following address hj28981.glddns.com:51820 and when I SSH into Slate AX and run the following it shows it as closed there.

root@GL-AXT1800:~# nmap -sU -p 51820 hj28981.glddns .com
Starting Nmap 7.80 ( https://nmap .org ) at 2023-07-08 22:48 CDT
Nmap scan report for hj28981.glddns .com (107.222.107.3)
Host is up (0.00030s latency).
rDNS record for 107.222.107.3: GL-AXT1800.attlocal .net

PORT STATE SERVICE
51820/udp closed unknown

Nmap done: 1 IP address (1 host up) scanned in 0.49 seconds