Multi AP function can be configured by navigating to Network → Wireless, deleting the pre-existing WiFi network (where the router is getting the internet connection from), and pressing the Multi AP button:
Once the configuration window opens up, enter the desired name for the Multi AP network, and press create.
Afterward, another configuration window will open up. Make sure to enable it, select the scan time (how often the router will look for the nearby wireless networks), and in the table bellow, SSIDs as well as the passwords of the wireless networks will need to be entered. The final configuration should look like so:
Press Save & Apply and the router should connect to a reachable WLAN network from the list after finishing the scan.