Has nothing at all to do with either NAT or the Firewall rules; it has to do with routing. Assuming your modem/router has this capability, all you should need

Has nothing at all to do with either NAT or the Firewall rules; it has to do with routing. Assuming your modem/router has this capability, all you should need to do is set up a static route pointing to the network/subnet that the devices behind the other router. You should not need a corresponding route on the other router because since it's on both networks, it already has a route to the other network. Even if it didn't, the default gateway on the router is the modem which can take care of routing the packets correctly. This is all assuming that the modem/router combo is assigning an IP on a private network/subnet to the additional router.