Sorry for multiple replies, just trying to add more info.
I did some more testing. I can open ports on my Asus and verify that it can get through externally to those ports.

So I know I can get the traffic in and pointed at the MT300N, I just can’t seem to get it to forward on to the device that is connected as a VPN client on the other side.

I have Public IP – which traffic comes in and gets forwarded to the port on the router @192.168.1.123 (Static IP assigned to the MT300N). I’m then forwarding on the MT300N to a device connected to it which is 192.168.8.214

Is it because that device is running via the VPN client which when connected is on yet another IP address that is the issue ?