Setting up Synology L2TP VPN with UniFi USG

Hi guys, I've been trying to set up my Synology NAS to do L2TP VPN but can't seem to get the ports open for L2TP VPN on the Unifi USG. Basically, I've been into the Port Forward properties of the USG under the Configuration tab and configured the port forwards as such:

And my WAN IN Firewall page looks like this:

Yet when I go to canyouseeme.org, ports 500, 1701, and 4500 are not open according to it.

I believe I have set up the VPN correctly on the Synology NAS because if I connect to the VPN on Windows 10 via its internal IP address, it connects just fine. I am trying to connect to the VPN via its DDNS name externally.

Any help appreciated thanks!