This is how to reset or perform the relearn procedure for power tailgate on 2018 mazda cx5. This can happen when your battery died or when you replaced you battery.
Remove the negative terminal. Install a charger go inside the vehicle and press the brake at the same time hold the high beam on for passing for about 1 minute. Or so. And after you can install the negative back and your tailgate should be working.
By the way pls dont forget to check all your fuses first before doing all this. Thanks