I have a Linksys WRT320N and I can't connect on the 5GHz band also.
If I disable wireless security, the DAP-1522 connects (but with no internet connection), with WPA2-personal and AES encryption I get a 4-way handshake fail in the DAP log. No way.
The signal fron the WRT320N is very very weak (a couple of brick walls in the middle, no more than 10 meters away), but with a Linksys WET610N in the same spot at least I can connect (but the connection is unstable anyway, so I had to go 2,4GHz with my older WRT350N, and with the WET610N, because the DAP disconnects from the network and "disappears" after a day or so).