I tested the KOPLAYER application and installed a few games and apps. The rotate function is working as expected. Before you use it, access the built-in Settings and change the display mode. In my opinion, the phone mode works way better than the tablet one.
If you still encounter issues, then simply use another Android emulator like Memu or Nox. Memu is faster than Nox, and the built-in features work way better, but that's on my PC. I don't know how it works on other PCs.
Also, you should leave rotation to Auto, because some apps can be launched only in a certain mode. Forcing them will cause errors or launch issues.