Separated 2.4 and 5gHz network SSID, now 2.4 does not show up?

Like many others, out wifi drops out ALL the time even with xfinity's 'new' moded/router.  In one of our many attempts to rectify the problem, I separated the 2.4 and 5 gHz SSIDs to different names and passwords.  The 5 gHz wifi shows it is available for wifi connection with a strong signal, the 2.4 has disappeared.  It was there briefly when I made the change and now nothing, even after resetting the unit.  Any suggestions?