Is it possible to put an ethernet card into the 2811 so that the cable can connect via cable modem and ethernet?
The ISP does not support constumer-supllied CPEs.
You've used all the built-in Ethernet ports?
You should be able to tell the Ethernet port connected to the cable modem to acquire its interface address from the modem using DHCP.
You might need to then NAT, if the modem only expects one device.