A USB dongle is highly recommended for SliceMK split keyboards as it significantly improves the keyboard's battery life (read about why).
This Raytac USB Dongle is the recommended option for the majority of users due to its compact size and professional enclosure.
This dongle is specifically sold for use with SliceMK split keyboards. This is not a generic Bluetooth dongle.
Other Dongle Options
The ErgoDox Wireless keyboard is specifically designed to use this dongle. While other dongles may work from a technical point of view, they are not officially supported. If you decide to use an unsupported dongle, configuring the firmware is entirely your responsibility.
Please note that while the same Raytac dongle may be available from other vendors, their version may be preloaded with an incompatible bootloader.
Users who wish to implement advanced firmware integrations that require access to GPIO pins and/or Serial Wire Debug should consider the Nordic nRF52840 Dongle.