@in-france
There are many many reports about the "connecting to assisting Nest device" error in these forums.
In the past, Google Nest used to have us set up our Nest cameras and doorbells by selecting our Wi-Fi network and entering the password. Then they decided--I guess--to make it "easier" for us by getting that Wi-Fi configuration data from one of our previously installed Google Nest devices. So an "assisting device" is just another Google Nest device that is already working.
The problem is that the process doesn't work very well, and your new Google Nest cameras for some reason can't find the "assisting device", and as customers, we have no idea which one of our already-working Google Nest devices is regarded as the "assisting device".
Some customers in this forum have reported success with either of these two work-arounds:
1) Temporarily turn off or disconnect your existing Google Nest devices (like your existing wireless camera) while setting up your new Google Nest device; hopefully, this then causes the device to let you select your Wi-Fi network name and enter the password (the way it used to work in the past). Then turn your other Nest devices back on.
2) Temporarily set up another "home" in the Google app, add your new Google Nest device in that home, which--since there are no other Google Nest devices in that home, hopefully causes the device to let you select your Wi-Fi network name and enter the password. Then, once the Google Nest device is working, remove it from the temporary "home", add it to your regular "home", and delete the temporary "home".
We have not encountered the "assisting device" error, so I don't know whether either of these will work for you.
Google Nest has not admitted to any of the above, and--in spite of hundreds of "cannot connect to assisting device" posts in this forum--has not fixed the problem or offered an official solution.
