You can't use that router with UniFi on stock firmware.
The only way to use that router with UniFi is by flashing the firmware to OpenWrt.

There are several hardware version for the DIR-825.
What hardware version is yours? The hardware version is usually written on the sticker at the bottom of the router.

DIR-825 Hardware Versions:
A1,B1,B2,B1/2,C1,D1

You have to revert to stock firmware first before flashing OpenWrt.

DIR-825 DD-WRT WiKi: Scroll down on that page then you'll see how to revert to stock firmware.
http://dd-wrt.com/wiki/index.php/D-Link_DIR-825

Once you're back on stock firmware, you can flash to OpenWrt by reading this guide:
http://wiki.openwrt.org/toh/d-link/dir-825

That depends on which do you want to use.
Barrier Breaker (BB) if you want the latest firmware, but it's still in trunk version.
http://downloads.openwrt.org/snapshots/tru...hfs-factory.bin

Attitude Adjustment (AA) 12.09 if you want the latest stable firmware.
http://downloads.openwrt.org/attitude_adju...hfs-factory.bin

With BB, you will have to install LuCI GUI by yourself.
If you want a firmware with pre-installed LuCI GUI, you'll have to use AA.

Once you've flashed an OpenWrt firmware, go to "System > Backup / Flash Firmware" then "Perform reset".

Once the system reset is completed & the router finished rebooting, you can follow this guide:
http://klseet.com/index.php/tl-wr1043nd-ve...unifi-10031-rc6

That guide should be useful for D-Link DIR-825 too as TP-Link TL-WR1043ND uses the same chipset.
Although I can't vouch for this as I don't have any DIR-825 laying around to test.

Do report back if you've succeeded.
Good luck.

I can't help much as I never had DIR-825 to test.
But according to the post in the link below, he had success in using DIR-825 with UniFi & IPTV on OpenWrt firmware.
https://forum.lowyat.net/topic/1734440/+13

Perhaps tch9 can help you.