Distance is not a problem on VPN connections
You don't need to install anything on routers, just configure incoming traffic and send OpenVPN incoming traffic to EFW.
You should configure your internet router to forward OpenVPN ports (TXP&UDP 1194) to Endian, or set router to pass all incoming traffic to EFW box.
The important point is that your endian firewall receives the incoming OpenVPN traffic.
If you have no access to configure anything on routers I'm sorry but I don't have any other ideas.
Another important point is that at least one gateway must have public static IP or dyndns.
Hablo español así que si necesitas más datos no dudes en preguntar
Saludos.