Reporting that I found the solution to this problem. When I updated the Intel AC 3160 WiFi adapter's driver from the Dell Support site, the problem was fixed. When you download & install this latest driver and look in Windows' Device Manager under Network, you will see the driver's version as 18.33.17.1.  If you ask Windows to update the driver, it probably will tell you that the latest driver is already installed.  It is necessary to download the driver from the Dell Support page for your machine and have it install the latest driver.
Having upgraded the Xfinity Internet Gateway device to the new XB7 unit, I now do not see my WiFi SSID.  All other units that we use in the house can see the SSID, but not the Inspiron laptop. Has anyone else seen this issue?  Does anyone have any suggestions of what to try to fix this?
Inspiron 5755 WiFi cannot connect to latest Xfinity Gateway