If you left the linksys in firewall mode (as opposed to gateway) it will likely block your connection to a PLC. You can configure it as a gateway, but then you would have to point to the new subnet with your default gateway on the PLC network or directly to the linksys as the gateway. What you may want to try is to connect the 2955 switch to the switch side of the linksys, i.e not the Internet labeled port. Then you will have to make sure the DHCP side of the linksys won't interfere with anything on the PLC network and that the addresses are on the same subnet.
I have not set up any like this, but it should work.
This is one of the reasons the Cisco AP's are more suited for this kind of environment, but I have a few of the linksys myself.
Randy