Bluetooth keyboard question

I have an Asus Chromebox-M031U, and often use a bluetooth keyboard that's connected via a USB dongle like this:

But as the Chromebox has Bluetooth 4.0 built in, I'm wondering do I need the dongle - or can I free up a USB slot?

I went into LibreELEC Settings>Services>Bluetooth>Enable bluetooth, then...

LibreELEC Settings>Bluetooth> and put the keyboard in discovery mode. LibreELEC found it, and I selected "Trust and Connect" which worked and it showed as connected. BUT if I remove the dongle the keyboard isn't connected.

Am I doing something wrong, or is the Chromebox's built in bluetooth not meant to connect to a keyboard in this way?

No joy yet. I have unplugged dongle, turned Bluetooth off in LE settings, rebooted, turned Bluetooth on in LE settings and clicked the 'connect' button on back of keyboard to put the keyboard in discovery mode...but now I don't even see the keyboard at all in the Bluetooth pairing options (but do see a different bluetooth device I have appear in the list, so that confirms that Bluetooth is indeed on on the Chromebox).

Any ideas, why this keyboard doesn't connect without the dongle? Or what log should I post if that would help diagnose?

The Bluetooth keyboard I'm trying to connect is made by Asus, and came shipped (along with a bluetooth mouse & the dongle) with the Chromebox. I the model number 90MS0000-P00010, and it looks like this:

OK, so I've been told that this keyboard connects with RF, not bluetooth! So that explains the situation!!

Thanks nickr and Irusak for your help.