Hello rtv-it,
Welcome to the forum.
When connecting to the Server 2016 hosted VPN, are you connecting from another client device directly to the 2016 Server? If so, do you have any other VPNs running? Do you have the port for the VPN to the server open when you have the firewall filtering traffic?
It may be worth running a packet capture from the router’s support.cgi page
when the VPN can connect to see what ports it’s using to connect and make sure you have that port open on the Balance’s firewall.