A replacement receiver may need pairing
Simply plugging in another LIGHTSPEED receiver may not be enough. The headset and receiver need to enter pairing mode at the same time.
Put the receiver into pairing mode
Connect the compatible LIGHTSPEED receiver to USB and use a thin pin to press the small pairing/reset button. Its LED should begin flashing.
Put the headset into pairing mode
Turn on the headset, carefully remove the left ear pad and locate the small pairing opening under the driver area. Press the hidden button gently with a pin.
Wait for the connection
With both devices in pairing mode, wait roughly 10–15 seconds. The flashing should stop once the link is established.
If it still does not connect
Repeat the procedure, try another USB port, charge the headset and make sure the receiver is actually compatible with PRO X 2. LIGHTSPEED is not the same system as Logitech Unifying.