Thanks for reaching out. I know how confusing it is when the others went offline right after you added your other Nest Protects. Let's see what we can do here. To confirm, how far are those 6 Nest Protects from your Wi-Fi router? Were there any recent changes to your Wi-Fi settings? What troubleshooting steps you've done so far?
Give these steps a try:
Let us know how it goes.