This page shows the list of clients connected to the router via WiFi.
When you first come across the page, you will notice a table of all connected devices, you can click on the blue icon to set an identifier for a specific MAC address. This will help you identify the device in the future.
Network Settings - WiFi - Clients
You can use the red block button to kick a client off the WiFi network for a specified time period.
It’s also a good idea to keep an eye on which device is using which WiFi band since some very old devices perform poorly on the 5Ghz band.