I'm going to be walking down this path once Unifi is available for subscription on my area. To answer your question, I have done some research. To allow DIR 825 to be used with Unifi, it must support Virtual LAN tagging. Unfortunately, the standard firmware doesn't come with VLAN support eventhough hardware wise it is supported. Only way for you, if you are adventurous, flashed the firmware to use DD-WRT firmware.

I hold no responsibility over this recommendation and thus don't complain if it breaks your router. I'm going to do it myself but only once UniFi is in my area. Plus, I have another spare router lying around if this really breaks my router. But DD-WRT current firmware for DIR-825 should be stable enough.

I will post my findings once I'm down this path :-)

Happy experimenting!