Thank you FurryNutz, those instructions worked. I can now see my Sonos device, which is plugged into the 3801HGV, via the AP.
One problem though... now I can't access the AP (dir-655) to enable the wifi. I changed the gateway IP to 192.168.0.50, as instructed. If it helps, the 3801HGV's IP is 192.168.1.254. Did I do something wrong??