Can you give us some more detail? What kind of wireless client are you using to try and connect on the 5GHz side (operating system, supplicant, and wireless NIC info)? You might want to try and disable your AP 2.4GHz radio to ensure that your client isn't preferring one radio over another. Depending on your NIC/supplicant capabilities, you may want to disable the NIC's ability to connect to 2.4GHz networks).

Since you have the 5GHz radio set to use dfs, do you know if it's actually selecting a channel? DFS can take a few minutes to find a channel after the AP comes up. Try hard-coding the channel.