SBG8300 no 2ghz or brief then out 2ghz SSID

Are you using the combine ssid option? 

If you do, check with a ssid scanner (not the built in one) to see if the 2.4ghz stays broadcasting with that enabled. 

Does the 2.4GHz network disappear on any particular device or all devices? If it disappears on all devices, does the 5GHz network also disappear?

Plemans, the way I found to scan wifi is using an Android device with a wifi scanner that can monitor signal strength and channels. Very helpful when configuring in a neighborhood or building with competing routers. But to answer, the SBG8300 stops broadcasting  on 2ghz. The SSID acts like it  is hidden. So all devices do not see it, but technically could connect as long as the device can connect to a hidden network. Most of my cameras, garage door opener, door locks, all go offline. After going into the SBG8300 and attempting to turn off then back on again ghe 2ghz for hidden/not-hidden and monitoring it to see if it will broadcast again, when it eventually shows up on a acan, them the devices work again. The most frustrating thing is that if you simply search SBG8300 2ghz and either problems or stops broadcasting there are too many posts on thos, it appears to be common, and for those of use using i o t devices we need 2 ghz. So to the other post from moderator, I hope I ansered your question, the SSID for 2ghz coming out of the SBG8300 cannot be seen by any devices. All devices can no longer see it. . 

moderator, the 5ghz SSID continues  to work with  no problems. And as phones, TVs, tablets, laptops, computers can use 5ghz most people might not notice any problems. But ALL of the smart home devices like switches, lights, locks, cameras, leak monitors, water valves, and literally everything else uses only 2ghz, and when that stops working the whole house  stops working. 

the 5ghz does not disappear.