Is there any option to change the wireless encryption type of the Nest Protect?
If yes, please try to fix the wireless encryption type as the most secure WPA2.
In addition, is there any error message when you fail to connect the Protect? Please find the link below for more details:
https://nest.com/support/article/Troubleshooting-adding-Nest-Protect-to-a-Nest-Account-or-Wi-Fi-network

I read info below on google. but how do I set different namens for 2.5 and 5 G networks within de deco app? I have tried all other tips but still not able to connect nest protects --------------------------- If you can’t add your Nest Protect to the Nest app, this can be a result of incompatible settings on your home Wi-Fi router/access point. 2.4GHz and 5GHz network names If you have a dual band router with both 2.4GHz and 5GHz bands, make sure they have different names so that you can correctly connect your Protect to the 2.4 GHz network. Nest Protect can’t connect to a 5GHz network and may not be able to tell your networks apart. How to use Nest products with 2.4 and 5GHz Wi-Fi networks