Operating Door Locks from Outside the Vehicle
Mechanical key

If you lock the driver's door with a mechanical key, all vehicle doors will lock. If you unlock the driver's door with a mechanical key, the driver's door will unlock and the passenger doors will unlock according to the current two press unlock setting.
Once the doors are unlocked, they may be opened by pulling the door handle.
When closing the door, push the door by hand. Make sure that doors are closed securely.
Remote key

To lock the doors, press the Door Lock button (1) on the remote key. Press the Door Unlock button (2) on the remote key, the driver's door will unlock. If you press the Door Unlock button on the remote key again within four seconds, then all the doors will unlock.
Once the doors are unlocked, they may be opened by pulling the door handle.
When closing the door, push the door by hand. Make sure that doors are closed securely.
Information
Immobilizer System
Smart keyHyundai Elantra AD (2016-2020) Owners Manual: Immobilizer System
The immobilizer system protects your vehicle from theft. If an improperly coded
key (or other device) is used, the engine's fuel system is disabled.
When the ignition switch is placed in the ON position, the immobilizer system
indicator should come on briefly, then go off. If the indicator ...
Hyundai Elantra AD (2016-2020) Service Manual: Rear Door Module Components and Components Location
Component Location
1. Rear door module
...