Caution: Do not install /e/OS on top of a higher version of stock OS. Before installing /e/OS ensure your device has the latest stock OS of the same version as /e/OS. If only lower versions of the stock are available then install the last available lower version before installing /e/OS.
As a general answer you need up to date firmware which as far as possible will be upgraded with manufacturer updates, so you really want the correct foundation imho.
Caution: Before following these instructions please ensure that the device is on the latest Android firmware available for your device. For example if available, before flashing /e/R install Stock Android 11, before /e/Q it should be Stock Android 10 or before /e/ Pie it should be Stock Android 9.
So I think the question that remains is … did your manufacturer offer an Android R firmware update?