Hello,
With residential modems configured to allow a single IP path, connecting a dumb switch behind the modem is not going to be practical as all devices will end up fighting for the one WAN IP and will just cause conflicts. The only solutions would be to install a router behind the modem and connect the devices up in that room and run a cat cable over to the other room and install the dumb switch there. If WiFi coverage is an issue where the modem is, then reverse the set up and you would need to run 2 cat cables to install the router in the other room and run the return wire back to install the switch in the room with the modem to cover the remaining items. It may be pricey but you could also convert the account into a small business account. They allow multiple paths on a single modem. You can have up to 8 paths on a single modem so a dumb switch could be installed behind the modem and installed the way you would like to install the way you would like to.