A new and unactivated version of Cubase will activate as the latest version available at the time of activation. If you bought a boxed version of Cubase 10 Pro and activated the licence after Cubase 10.5 Pro was released, you get Cubase 10.5 Pro. If you activate in the grace period before a new version is released, running Maintenance in eLicenser Control Center after the new version is released will automatically update your licence to the new version.
However, you don’t get free updates for life! If you activated your Cubase 10 Pro before the grace period for Cubase 10.5 Pro started then Cubase 10 Pro is all you get without paying to update.
There is no point attempting to reuse a Cubase Pro activation code - it is a one-time code. Run Maintenance in eLicenser Control Center with the USB eLicenser containing your Cubase Pro licence plugged in. If the licence stays as Cubase 10 Pro, then that is all you are entitled to without paying to update.