I used the Reboot to CoreELEC (Reboot to CoreELEC APK) to install CoreELEC. Just make sure, under Android, you select “Enable developers options”, “OEM unlocking” and enable “USB debugging”. That works for me. Using Dune HD Pro 8K plus.
Keep in mind, the R3 or R4 remotes do NOT work under CoreELEC. I use the Ugoos UR02 remote, it is fantastic, and CE has drivers for IR and Bluetooth.