Try a fresh install. Download the game again, delete the folder (you can keep the saves). Make sure no artifacts remain that could interfere. The BepInEx plugin works fine for me, so IDK what could cause it on your end. Try setting both Locale AND Region to Japanese.
That is exactly what i did.
I downloaded the full install, copied my old saves and my KeyConfig.json.
Then i took the BepInEx_x64_5.4.22.0.zip and put all files from that in the new folder, started the game once, closed it again and then moved the HotM_cheats.dll in the Plugin folder.
Then, after the cheats did not work and i noticed the message at game start, i downloaded the update and moved the files from that in the game folder (hoping it would be a different Assembly-CSharp.dll), same result.
Maybe as additional info, this is not my first time using the plugin.
I have 2 older versions of the game still on my hard drive with the plugin and these still work totally fine with the cheats.