So just to be clear, you have multiple cameras that have been working fine for maybe 5 months or so? Then they all fell offline and are refusing to reconnect? But nothing obvious had changed? And the Wifi & Internet still work fine for other devices?
If so, as you go through the excellent checklists above, I would say to look for the single point of failure that could affect everything at once.
Also, anything non-obvious that may have changed, like environmental stuff - neighbors flooding your space with same SSID WiFi, moving a bookcase or smart TV in front of the router, etc
( the next two in no way explain why the cams dropped offline in the first place, they’re just bumps I’ve hit connecting cams )
As @tomp and @Brlepage said, the band issue can be really problematic. Especially if they have different passwords and a modern phone connects to 5.0 and tries to give that password to the cam.
On the QR scanning, that used to give me fits until I learned to do it in a very dark room and preferably right next to the router for max signal. Once it’s all setup and appearing in the app, then I put the cam where I want it.
It will be interesting to hear what happens!
/Edit
One more thing to check under ‘things that have changed which you may not immediately suspect’ - did you allow the cams to update their firmware? I have had the automatic firmware update gone wrong kick a few of cams offline