What should I do if the car rental agency is not working when I want to return the car in advance ?

Check if there is a self-service return car service, fill in the form and put the keys in the drop box

Just put the key into the box at the door.

Try to return the car during working hours, it's better to check the car on the spot, otherwise it's hard to say later if the car has some damage. There are also those who self-drive and throw the car key in the wrong place, even in the trash bin; there are also those who say their car is broken and the car agency directly deducts the money.