No need to return the router and buy a Ethernet switch, as the router can be used as an Ethernet switch if needed. Just turn DHCP off on the router and only connect the LAN ports, leave the WAN port disconnected, and you have an Ethernet switch!
This also assumes the DSL modem supports and is configured for routing, some do, some don't. I assume in the current configuration the DSL modem may be configured to PPPoE, I have not researched the specific DSL modem we are working with here, yet.
JamesO