There's no way of removing devices via the Router page unfortunately, you have to do all the removing/renaming of devices via the app for some reason
in the app go to:
Manage -> WiFi Controls
Once it has finished loading all the sections with the green loading circles:
Then scroll down to Devices, and it should list connected and disconnected devices that have been on the network, you can scroll to the right for more devices.
Click on a device you want to remove, then click the three dots at the top right of the page, and click on "Forget this Device"
Repeat this process for every device that you want to remove.
