@Fen 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 the help, MetalGlacier.