It looks like that USB-BT500 is using a Realtek RTL8761B chip.
I have a different USB BT dongle that also uses the same RTL8761B, working in CE NO. If you are having problems getting a remote to pair through CoreELEC settings, try it through SSH instead.
CoreELEC:~ # bluetoothctl
Show all BT controllers
[bluetooth]# list
Set the USB BT controller
[bluetooth]# select <bt-controller-mac>
Put your remote in pairing mode
[bluetooth]# scan on
[bluetooth]# pair <remote-mac>
[bluetooth]# trust <remote-mac>
[bluetooth]# connect <remote-mac>
You can add this line to your autostart.sh to select the USB BT dongle on startup
bluetoothctl select <bt-controller-mac>
Alternatively you can blacklist the driver for the internal BT controller. My experience is that this is a worse option because without the BT driver, the internal BT controller may not be able to go into a low power mode.