Your setup should look like the following
Modem > router > switch (if any) > PCs , wired or not.
Then make sure that all PCs are on the same subnet. You should only be using one router and one DHCP server if you are using DHCP. Static IPs can be used instead.

Any router has a WAN port. That gets connected to the modem. At least that's the way I like to do it. It makes things a lot easier.
It doesn't matter if the router is wireless on the LAN side and it doesn't matter if there are servers on the network of if they are all workstations. The only server I have is a FreeBSD FTP and SSH server but it's connected the way I described. It's an in house server only. In other words, no one from the outside can login to it. I use the same setup that I described with or without a server.