So you have a VPN installed on the router. Which means the router is running a VPN End Point, correct?
Which means that any PC connected via the VPN to the router End Point is effectively on your home LAN.
Which means that the PC connected via the VPN to the router End Point should have an IP address supplied by the DHCP server on your LAN, in the address range used on your LAN. Does it?
You should be able to ping the PC running MC just as if you are on your LAN, using a Windows Command windows and the simple "ping 192.168.xxx.xxx" or whatever address range your DHCP server uses. Can you?
If the above conditions work, try accessing Panel via the MC Server LAN IP Address directly, rather than using the Access Key method. Something like;
http://192.168.xxx.xxx:52196/Panel/#/browseNote:
You don't need to open the MC Server port if you will always be using the VPN connection to access Panel.
I'm pretty sure Panel uses more than just one port number anyway. But I guess if it works without the VPN and 52196 opened, and forwarded to the MC Server I presume, then Panel must be opening any other ports required from within your LAN, just like any request to the internet from inside the LAN opens ports as required for the requesting process.
There will be a firewall on the MC Server as well as on the router. But if you can connect to Panel while connected to your LAN at home, then that firewall should also allow you to connect to Panel when using the VPN.