I have added 2 port forwarding rules in the G3100 router, both are enabled. One is to a camera app to forward udp port 8xxx to camera at 192.168.1.xx1. The second is to open an OpenVPN udp port 1194 to a Synology NAS at 192.168.1.xx2. When I check those ports from an online port checker using my computer connected to the G3100, the 8xxx port is reported as Open while the 1194 port is reported as Closed. Does anyone know any reason for this? Of course, the camera app works from the internet while the OpenVPN client never connects.
What port test site are you using? Make sure the test site can test UDP ports. Since UDP is inherently different from TCP, which requires acknowledgement, we can only determine UDP open (or closed) ports passively.
The best way for you to test your external ports is through port scanning, such as using nmap. You need a computer that can reach your router's WAN port from outside.
This Internet Common Ports Probe attempts to establish standard TCP Internet connections with a collection of standard, well-known, and often vulnerable or troublesome Internet ports on YOUR computer. (grc.com)
If they only use TCP to probe the ports, how could it detect the UDP status on the port.? L2TP requires two UDP ports. Even if those two ports are open, they are probably not being detected by TCP probes sent by the site.