UK Car Repair Questions? Ask a Mechanic for Answers ASAP
It sounds like your immobilizer is activating- which should also be indicated by an icon on the dash of a car with a key in it.
Try disconnecting the battery again, touch the cables together to discharge all modules and reset them, then reconnect the battery. After this lock and unlock the Cayennne a few times with the remote to recognize that the system is operational, and try the key in the ignition.
If it has completely forgotten your key adaptation you will need to get it to the dealership so they can re-adapt the keys (and you will need all keys present at that time)
Did you try the process I recommended? Did you ever resolve this issue?