D-Link router lost a network

Please explain.  How are you observing this?  What do you mean by "one of the networks"?

When I click on the available Wi-Fi networks. A network that I used to have just disappeared

"When I click on the available Wi-Fi networks": on the D-Link or on some other device?

"A network that I used to have just disappeared" was the D-Link router providing this network?  If so, log into the D-Link and see how the WiFi is configured.

I've checked all the settings and can't find a problem

"I've checked all the settings ": on the D-Link router?

It would be helpful if you posted screenshots of the settings with regard to WiFi.

For example, have you confirmed that the SSID broadcast is on?  If it is, turn it off, save the setting, and then turn it back on.

Thank you for the update.  Have you confirmed that the WiFi is disabled on the D-Link?  You likely don't want WiFi coming from both the D-Link and from the Xfinity router.

Yes.  Thanks