I like to see, that Ubisoft added achievements here in Steam for some old titles.
And for AC4 Black Flag you can still buy Gold Edition.

But AC2 Revelations - it's pain.

You want to unlock all achievements, but you don't have DLCs?
Let me try to help you.

I expect you to have at least Standard Edition of this game in Steam.
This instructon won't work without any AC2 Revelations license in Steam.

To check just run the game, and you will be see list of your licenses.

If you can't see it - right click on game in library -> manage -> keys
You will see your keys.

So, if you have Gold Edition, or you have "Lost Archive" in list of available DLCs - good for you.

Read this instruction to install DLCs: https://steamproxy.net/sharedfiles/filedetails/?id=3208375262

This is my situation.

You don't have the right key to unlock everything.
You can't buy it in Steam and in Ubisoft.

But here comes the re-sellers.
Are you so desperate as me, so you will waste some money, so you can unlock 100%?
I'm not fan using re-sellers, and I don't want to make an ad for them.

I was lucky enough to find and buy Gold Edition on some random sale.
The key I got was for Ubisoft, not for Steam.

That's fine - go and activate it in Ubisoft.

So, how to unlock achievements in Steam, if DLCs unlocked in Ubisoft?

1. Run AC2: Revelation from Steam
2. Ubisoft app will provide you option to run Standard or Gold Edition -> Choose Gold
3. Unlock your DLCs achievements - they will pop up in Ubisoft, but not in Steam
4. Once you done with achievements (or some of them) - close the game.
5. Run it again from Steam
6. This time choose Standard Edition in Ubisoft -> it will unlock achievements for Steam as well

Ubisoft did great job of adding achievements to Steam for old titles (if it was Ubi).
But it did worst for all players without any good opportunity to install or to buy DLCs.

I feel scammed. And the ignorance of Ubisoft... I guess, it's not a surprise.

I hope that this guide will help you, if you're so desperate as me.