I am currently trying to create a route that will allow me to create a sub-network controlled by another router.
Right now, my MX is 192.168.1.1, the gateway (another router) is plugged into it on port 4 with WAN IP 192.168.1.200.
The subnet controlled by the gateway is 192.168.100.0/24, the gateway itself has a LAN IP of 192.168.100.1. I am trying to be able to ping both ways between my MX network and the gateway network.
Is there a way? I tried a static route (next hop to gateway WAN IP (.1.200)) but it just renders the device unaccessible from either WAN nor LAN IP.
On your MX add a static route for 192.168.100.0/24 pointing to 192.168.1.200 - so you have that bit correct.
Is by chance this other gateway a firewall or performing NAT between its LAN and WAN IP? You will need to stop that it make this working.
It may be simpler to take over the 192.168.100.0/24 on the MX, and get rid of the other router.
In that case your options are limited. The 192.168.100.0/24 subnet will be able to talk to 192.168.1.0.24 via NAT, but 192.168.1.0/24 will not be able to initiate traffic into the 192.168.100.0/24 subnet.
You could put the MX in both subnets (disabled DHCP for the 192.168.100.0/24 subnet). Then for the one client statically configure it with the gateway of the MX for the 192.168.100.0/24 subnet.