This happened to me a while ago, the simplest way to reconnect them is by factory resetting them and then setting them up once again. Annoying but its they only way I got mine to work.
Here's how to reset it: support.google.com
@All thanks for posting here in the community. I'm sorry to hear if your Nest Protects won't connect to your Wifi. The Nest Protect can't connect to 5G Wifi network and can only be set up and controlled through the Nest App. Follow this link on how to update Wi-Fi settings on Nest Protect.
In case the issue persists, give these steps a try:
Looking forward to your response.
I appreciate you sharing the information, MetalGlacier.
Same with mine. They previously worked but I was silly enough to get a new Google wifi pro mesh system thinking it would solve some of my problems (it did) but now 3 of my 6 protects won't connect after scanning the qr code, entering the 6 letter code on the back of protect, factory resets, network info "forgotten" and every else I can think of. Note: i created the same "wifi network name" but changed the password to remove some of the riffraff. Would be nice if you have 20+ Home/Nest devises and 40 or so other devices to update the app with the new password and all devices would update but noooooo. That happened with some small apps but not Google. Shame. We need Matter ASAP unless that's also a disaster.