This sounds like a WiFi keyboard not a Bluetooth keyboard. 2.4 GHz is a WiFi band not within the Bluetooth range.
You don't need a dongle if it's a real Bluetooth keyboard. Dongles are needed if the connection is proprietary as an example Logitech makes such a keyboard for managing multiple systems in a computer room. It requires a driver to work properly.
Sadly Logitech doesn't offer macOS support for their Unifying keyboard. mouse or trackpad devices [https://support.logi.com/hc/en-us/articles/360025297913-Logitech-Unifying-Software|Logitech Unifying Software]