@bluefire3648,

Unexpected Behaviors like this are most often the result of a setting within the router that is conflicting with either your connection from your ISP, or your environment. I would first recommend trying to change the channels and maybe even the channel width of the 5Ghz band. There are specific parts of the 5Ghz band that are only allowed for consumer use when there is no other traffic on the frequency around. Routers will automatically switch channels when a conflicting signal is found. 

If you use a Wi-Fi analyzer app on your phone, does the current channel show as being overly crowded?

@bluefire3648 

Which channel did you use?

How many devices are connected to the 5G network, and what are they? How about the 2.4G and wired connection, are they both stable?

  @bluefire3648,

Channel 120 in the 5Ghz spectrum is reserved for communications of weather stations, all routers and APs are required to shut down their radios if there is another signal detected on the same channel. You will want to look up DFS channels to see what other channels, may have the same limitation; as different frequencies are used for different things such as traffic management or emergency signals. 

If there is no signal while you are configuring/starting your network; you will be allowed to activate the 5Ghz network; however when a conflicting signal from an official source is detected, the router or AP will shut down the band, and begin performing a channel scan. Only after which will the network begin broadcasting. This can often take up to 20 minutes to fully recover from something like this occurring.

Try trying other channels for the 5ghz network to see if this is happening to your network.