is your modem just a modem or does it have a router built in if it's also a router then you're nated twice and you have to set up port forwarding on both devices
set up as many separate points where you have to manually set up the device so they act as firewalls stevey
that's what a firewall is, a bunch of devices set up in a chain to block people from using the network at all, try to make the process as inefficient as possible