Handedness: Some microprocessors have problems detecting which board half is connected to the PC based on voltage. This can be resolved by adding SPLIT_USB_DETECT to keymaps/<mykeymap>/config.h. See:

• https://github.com/qmk/qmk_firmware/blob/master/docs/feature_split_keyboard.md#hardware-configuration-options
• https://deskthority.net/wiki/Elite-C
• https://medium.com/@keebio/the-case-of-the-wayward-elite-c-73f0fd691f88

LEDs: The LEDs may not display correctly if an earlier setting of the MCU has them turned off. This could happen if you had earlier used that MCU for a different board. Getting the LEDs set back to on requires accessing the toggle button on the adjust layer. To do that you need to press the two thumb layer keys and the RGB toggle button. If this seems to be the issue, it may be easiest to solder the two thumb layer keys and then use tweezers to 'press' RGB toggle key. It is also possible with three wires or tweezers.