actually, @eduperez was being more precise than I was. They are very similar things, and I simplified. But the definition from @eduperez is better.
The point here is that we have established that OpenWrt can indeed override/clone MAC addresses, but the remaining issue is really not related to OpenWrt (i.e. how can you get your game working again -- we cannot know that because we don't know how the game is detecting your anti-cheat thing, and that is out of scope of OpenWrt).
Therefore, I believe your actual question/problem is now solved based on the actual title and first post, and the relevance to OpenWrt.
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.
Thanks! 