Hello Bill Hornaday,
Your specific Modem/Router combo does in fact support both 2.4 and 5 GHz. Your older device must have offered both the 2.4 & 5 GHz networks. Newer devices only show up as one network allowing your device and access point to determine if 2.4 or 5Ghz would provide the best signal.
From Windows OS you can determine if you are connecting to 5GHz network by doing the following:
Click the WiFi icon in the bottom right). Click on “Properties” of your WiFi network. In the window that opens, scroll down to “Properties”. “Network Band” will either say 2.4GHz or 5GHz.
You can download an app on your phone to analyze what networks you are broadcasting.