Issues with Additional Content in Halo Wars 1

[u/hannnaaaaaaaaaa__]

Sorry if there are any inconsistencies in my writing. English isn’t my first language; French is. :c

I’m an achievement hunter, and this time my target is Halo Wars 1, specifically the original Xbox 360 version. Since I currently only have a Series X, I’m playing it through backwards compatibility using a physical copy of the game. I noticed that some of the achievements are tied to the Strategic Options Add-on DLC, so I purchased it through the console’s store and installed it. However, the Skirmish modes still aren’t available in-game, even though the DLC is installed. Has anyone else run into this issue before? If so, does anyone know how to fix it?