Ashley's answer is very good.
Alternatively (if there are issues putting both networks in the 192.168.61.xxx range from your router), you can try choosing an address outside the dhcp range of your router (likely something above 192.168.61.200, check or change router settings).
Or you can try to widen the ip address range of your computer and the ones attached to your Thunderbolt port by changing the subnet mask to 255.255.0.0. This should allow you to keep the 192.168.70.xxx IP addresses on your "middleman" computer and to manually assign the other computers in your net a 192.168.61.xxx IP address (again, outside the dhcp range which should be set to below 192.168.61.200). If you have a second router between the middleman computer and the others, that also needs the changed subnet mask, better with DHCP turned off and a static external IP address in accordance with your Thunderbolt network or your internet router (would have to test such a setup to be sure). If you don't, you might want to change the router address to your actual router (192.168.61.1), which the widened subnet mask makes possible, or leave the router out, if the system allows.