It sounds like one of three things is not configured correctly: your default gateway is not set right (obviously the WAP and wired router must have different IP Addresses), your DHCP Server on the WAP is not in the same range or subnet mask as the 5 PCs, or that you do not have routing configured properly if you are using it. I think the most likely source of the problem is that your default gateway is incorrect.
You probably have it wired like the following ASCII art, whereas Inet is the Internet and CM is the cable modem.
In this scenario, you’ll want all clients to point to the IP Address of the Router and let the router determine if the traffic needs to go out the WAN interface (CM interface) or back to the local LAN; that is, if you’ve broken down the LAN into multiple subnets.
You haven’t mentioned pinging between these systems at all. If you can successfully ping even when the systems on the WAP are set up for DHCP, then you may have a firewall on the WAP configured to block the standard MS ports for SMB/Netbios and MS Directory Service.
Hope this helps.