How to add a static route to AirPort Time Capsule?

I am building a virtual lab with VirtualBox to simulate a network with multiple servers, using a single physical host with 32GB of RAM. The host PC is coected to an AirPort Time Capsule for inteet access.

My goal is to create a virtual gateway server that acts as a router for all the other VMs in the lab. The NICs in the servers are all configured as inteal-only except the gateway server which is configured with 2 NICs; one on the same inteal-only network as the others, and one in bridged mode. The inteal-only NIC has a static IP of 10.11.12.1/24 and the bridged NIC is getting an address via DHCP from the Time Capsule. The gateway server is ruing Windows Server 2012 R2 and has the RRAS role installed and configured with the LAN routing feature only (no NAT).

My problem is that I caot route traffic out of the virtual network. The other servers on the network can ping the gateway server, and the gateway can ping them. But none of the other servers can ping the outside world except the gateway server itself.

I'm pretty sure I need a static route on the Time Capsule to the gateway server's address, but I don't know if that's possible on a Time Capsule.

Is there a way to add a static route to a Time Capsule? Or am I doing this all wrong.

BTW, all this works fine if I enable NAT on the gateway server, but I don't want to double-NAT to the inteet.