How can I upgrade router on a stick redundancy?

According to the spec sheet on the 1812 it looks like FE0 and FE1 are both WAN and all the rest are LAN ports, so you're using up all your WAN ports connecting one to the modem and one to the switch. I don't think it would be possible to connect a LAN port on your router to your switch in tandem with the line coming from the WAN port.