It can only act as a repeater if it is placed in the range of the current AP. The current AP also needs to have the ability to link itself with a repeater. Essentially, you’d configure both to talk back and forth by using their MAC addresses inside of the web GUI for both boxes.
If you have computers in the other buildings on the same network, I suggest using the 4-port switch + AP route. They have got to be connected to the wall 
Direct wireless through an AP is a lot more reliable/efficient than using multiple repeaters. They are prone to double the interference, due to the boxes having to talk with each other, as well as connecting to your wireless device.
What model is the Zyxel? If you cannot find the settings for it, I can see if it has the ability to use a repeater, otherwise you may have to disable its wireless and buy two or more additional APs to create the wireless network you prefer