Thanks Martin, it worked as you described.
I downloaded and installed Cubase 10.5 and then activated it using my Cubase 10 code emailed to me. You’re right, I could have entered the activation code either before or after. I found that when I did activate it I was automatically upgraded to Cubase 10.5 anyway and the download link in My Steinberg account was for the newer 10.5. All up and running now! Cheers!