Why do wifi networks disappear and appear, and become impossible to connect to?

You need to dramatically reduce the width of most the channels assigned to your 2.4 GHz band. ONE Router that wide hogs the ENTIRE 2.4 GHz band, and you have at least five of your own.

Provided your networks are spread out across that band, then your connections will drop less often.

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From where you are measuring, your 5 GHz signal is not very strong.

If you are using 2.4GHz, there are only 3 non-overlapping channels (1,6,11). If you have 5 or 8 networks each set to a different channel, they are overlapping for sure.

It would help this discussion if you provide specific details about your router & defined networks and the channels they are using. There is also the possibility of periodic contention from other WiFi access points that may be in your neighborhood. At my son's house we can see dozens of WiFi networks tripping all over each other. We had to set his network to a high 5GHz channel to get him a clean & reliable signal.

FWIW, here's a reference for WiFi channels -> https://en.wikipedia.org/wiki/List_of_WLAN_channels

JoshuaFox wrote:

They are on different channels. There is no contention, as shown by Inssider. On severalk Android, Windows, Linux devices, as well as this Mac until recently, no such problem occurred.

'different channels' is NOT sufficient, as each nominal channel spills up and down the spectrum and uses MANY adjacent channels as well as its nominal channel. Wikipedia has a chart you can use to examine the spread of each channel, depending on channel width and channel number, but it's complicated.

This "spectrum" graph from inexpensive Utility WiFi Explorer from the Mac App Store can show the spectrum used for each channel.

Most of these networks 'connect just fine' but are NOT working:

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JoshuaFox wrote:

How do I avoid this:?

By having only one wifi network at home.

JoshuaFox wrote:

They are on different channels. There is no contention, as shown by Inssider. On severalk Android, Windows, Linux devices, as well as this Mac until recently, no such problem occurred.

Well, if you are so sure about those 5 wifi networks, then reset the router that your Mac is connected to.

They fight each other to get through and in the end none will. Having one wifi network would also saves a lot of money. 👌

I haven't seen the reason for all those WiFi networks in your home.

this is more typical