I do not think it has DI.
It just says "7 stage RO system" and does not mention DI. I do not see it and if it does, it isn't the standard DI. Plus DI isn't used for drinking water.
Post carbon is for taste and ADDS STUFF BACK TO THE WATER... you do not want that for a reef tank.
You would want to add/modify this filter... you could add a T line and have one go for drinking and one for RODI if you wanted. So make RO only with the carbon at the end for taste for drinking and then a T to skip all those and go to "DI" for the tank.
but yes ... you bought a system for drinking water, not a reef tank. My husband made the same mistake when he gifted me my first RODI... the company actually had "DI" in the listing though even tho it was a RO machine so they ended up sending me a DI portion to add...

I never bothered to use the tank since I didn't care about on demand drinking water.
Basically you would not want to use the post carbon filter, the UV (it won't hurt but also won't be useful so no reason to pay for bulbs every 6 months/year), under sink container, and you need to add "DI" portion (I use two cartridges).
I would just return it if you can since it is going to be more cost to fix this. The only reason to keep it is if you want to also have drinking water. Keep in mind RO systems... since for drinking need filters removed and the tubing/housing portion bleached from time to time. I don't do this with reef tank only water. The bacteria isn't an issue for our tanks.