I would have to guess that the modem is doing NAT, otherwise there is no way you will get all the pc's online at the same time, unless you purchased a large block of public IP addresses from the ISP.
The only way you can do this, is to have a single network. Right now, from the sound of it you have 2 entirely SEPARATE networks. One where the AP is connected ONLY to the ISP, and the other where the AP is connected to ONLY the printer or print server....
What you will want to do, as CntdwnToExtn mentioned, is get a router. Have the router between the DSL modem and an AP, then connect the printer or print server to the router. That way, everything will be on the same network.