Hello and welcome to Just Answer!
The only way to disarm the security system without a key fob is to unlock either front or liftgate using the key. If the system won't disarm with the key there isn't anything else you can do besides getting the vehicle to a shop and have them program another key oem key fob.
I wish there was an easy way to do this, but if your key doesn't disarm and you don't have another working key fob, you will have to get another key fob programed and all the key cylinders need to be checked to see if the switches are working and if they are, you might have a problem with the central timer module. The CTM controls the theft security system.
I hope this helps, let me know!
Unfortunately, there isn't anything you can do to disable this if you can't disarm it. These new vehicles are designed so you can't bypass the security system once it is enabled from new. I sure wish there was a way to disable the security system, but there isn't. I know this isn't what you wanted to hear, but that's how these are.
Sometimes this can work if the CTM is locked up. You can try this a few ways.
Turn the key on, disconnect the battery for a minute, then re-connect the battery while the key is still on.
You can also try it the way you just said and see if this works. I can't say whether this will work, but it is definitely worth a try.
I can't say if the is a CTM issue or if the door key cylinder switches are all defective??
Let me know if this works, thanks and good luck!!