Just wanted to post some helpful info after scouring several forums to no avail.
On my wife’s 2018 Rapid spaceback the battery in the spare key had died, remote unlocking wouldn’t work but would unlock and start the car via the key in the door lock.
At the start of the process I had the main key fully working, and the spare not. Through trying various processes from forums and you tube I managed to get both keys out of sync, so neither worked to lock/unlock via remote.
After reading the manual, I bought two brand new batteries (CR2025). Working on one key at a time firstly the small red led now lit up with each button press (which it wasn’t doing previously) but at this stage the car still wasn’t responding. Enter the manual- followed the instructions (press any button on the fob, then unlock with key and this should re-sync it) this didn’t work for me.
This did-
unlock with key in drivers door, alarm went off as soon as I opened the door. Put key into ignition and START engine. I let the car sit for approx 1min with the engine running, turned car off, locked door with key.
Now I pushed the lock button on the fob, car responded with a Clunk. Pressed unlock, car now responds as normal, locks and unlocks via the fob and starts as normal. I followed this process exactly with the 2nd key and it worked.
Hopefully this may help someone else out there that may be struggling (as I was) contemplating a trip to a dealer to get sorted.
New Batteries.
Put the key into the car and start it and let it run for a short time.
Lock with key.
Push lock button.
Unlock with key.
Sorted.