Sorry for digging up an old thread, but did you ever solve your issue, I also have a Nest Protect that refuses to connect to my Meraki Network. Network is a L3 with Access Points in Bridge Mode, we also use advanced security on the MX. I see lots of Connects/Disconnects in the logs but no indication of blocked services.
I did take the unit home and connected right up to the home Wifi. Before I go adding a new SSID, Whitelisting, etc. I was just wondering if anyone has a configuration they know resolves the issue.