@Ryan5 on an MX you don't assign a LAN IP to a physical interface. The IPs are assigned to VLAN interfaces and these can be assigned to physical interfacea, when you put an MX in single LAN mode the IP is assigned to the default (only) VLAN and is available on all active physical interfaces.
As for the routing, if you have LAN subnets not directly connected then you need to add them as static routes on the MX and point to the next device in the chain. From that device you need to add a route to say the MXs networks are available through the IP you assign to the LAN. If your internet access it via the MX then just set a default route from the other device pointing to the LAN IP of the MX.
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