Solved: Nest protect not connected

[Rudi4711]

today I opened my Nest App and recognized that my devices aren't connected any more. I'm trying no since 3 hours to get them connected again and followed a lot of articles and howtos i found in the internet. I did the following:

1. Change of WLAN settings for device via App -> different error codes after several tries, most P014

2. removed all 3 connect devices from my home
  - Rebooted my AccessPoint and Router and enforded 2,4 GHz mode
 - deleted the app, rebooted my smartphone, deactivated mobile data
 3. Created a new home in Nest App and tried to add my 3 protect devices -> no I get Error Code P006  
is there a page where those error codes are explained in more detail? The help pages basically tell to reboot/reset everything, but this didn't work for me?
Has anybody out there an idea how I can bring my protects back online?

Moderator edit: Subject updated for clarity and findability.

[Unhappynestcus1]

I have been fighting with this for a month now. They did a firmware upgrade which caused the issue and made the mistake of following the debug steps are remove them from the account to add them back. customer service is just atrocious. They don’t know what the hell they are talking about and they refuse to acknowledge the problem. They tell me it’s my wifi even though it passes all of their tests and I cannot connect one nest protect per home structure but not 3 of them in the same “home”. Right now I have 3 different homes created under my account and I have one per home. The downside is that they don’t talk to each other but better than no connection.  I will not buy google products again. Apple rules!

[Rudi4711]

Thanks for your reply. I meanwhile managed to bring my Protects sort of online: I created a hotspot with my mobile and with this I was at least able to reactivate my Protect device in my 2nd home (didn't work with my first home ☹️) This page gave me a hint for this "trick":

https://www.googlenestcommunity.com/t5/Nest-Protect/Nest-protect-unable-to-connect-to-WiFi-error-cod...
However the situation is still a mess. I now have all 3 protects online again with a valid configuration and they talk to each other (good), but they use a hotspot created by my mobile (bad). They show up as online in the NEST App, even if my hotspot is not active (very bad - what does online then actually mean?). 
When I try to switch to my default wifi I still get those random not documented P0?? whatever error codes. I even added the MAC addresses of my protects to the 5Ghz MAC filter of my router to enforce the use of 2,4 GHz band - nothing works. 
So I now have my protects sort of working again, but the setup is in a very bad shape regarding all the online features of NEST. Very very disappointing 

[Rudi4711]

UPDATE: I managed to switch back to my default wifi by disabling the "smart connect" option in my wifi router. It can't remember when this was activated but I'm pretty sure it was active for a while before I recognized my Protects where offline. Nevertheless disabling "smart connect" did the trick for me

[Knoxoverstreet]

So for those of us that don't necessarily want to run the mobile Hotspot, what do we do? I have one that works and two that won't connect to wifi.