How to configure your WiFi for better security?

You would have to consult the manual for the modem, the supplier of the modem, the manufacturer of the modem, or another user of the same brand and model of modem. You haven't even told us what model you have, so we cannot advise you how to alter settings on it.

AT&T supplies several models of modems/gateways with wifi.

The gateway that AT&T supplied for my home service is manufactured by Arris and is a model BGW210-700. It uses an internal web server for configuration.