My setup is:
2820N
TP-Link WA801ND access point in client mode.
Popcorn Hour media box

I've connected the TPlink WA801ND to the 2820N, and am getting good signal across the local network from the popcorn hour (media box, torrent download etc.) I'm able to see shares and open up services in webpage etc (transmission...). Problem I'm having is the internet doesn't get through. The popcorn hour is set to get IP and DNS automatically, and the WA801ND passes this through OK, but nothing internet based comes through. Does anyone have a similar setup, maybe using slightly different kit, and have similar problems / solutions?

Did you set the default gateway address?? Have you tried any other wireless device to narrow it down to the repeater, device or 2820?

Might be worth setting the Popcorn device to fixed IP just to remove that uncertainty?

I had the same problem - only I am using the dlink dap-1522 bridge/access point to connect the popcorn device to my 2820 router. LAN access was OK but no WAN access.

The solution that worked for me was to delete the arp cache in telnet mode:

ip arp del <IP address for popcorn> LAN
ip arp del <IP address for popcorn> WAN