Only 3rd party optical link cable is the new VR Cover 5m premium link cable. Is that what you have? I have this cable and it works great. I still prefer my Q2 wireless with Air Link though. Maybe try disconnecting and reconnecting your link cable and try again. I occasionally need to do this with the headset connector.
Also, what are your specs and is this a laptop? LE2 requirements are pretty high. If you cannot get it to work maybe just get a refund from the oculus store.