This worked for me -
swconfig dev eth1 set enable_vlan 1
swconfig dev eth1 set apply
Then click run commands
I had the same problem with WN2000RPTv2 (same hardware and DD-WRT FW as WNR2000v3). I've set it up as a wired WiFi AP, and although clients connected perfectly over WiFi, there was no way to use the 4 Ethernet ports to connect computers over cable (they could communicate to the WN2000RPT router self, but no way to connect to the main router, to the devices on the main net, or to the external world, even with a static IP address). I tried everything possible, but did not find any solution until I saw the post of Anthonydoud and applied the commands. It really worked, so thank you very much, Anthonydoud! I wonder how you found out this solution?