We had a strange thing happen on an office turnup yesterday. The MX67 and MS 48 port went in no problem - registered and worked. The wired desktops and phone got out to the Internet and the tunnel formed to the data center. Everything was working fine - EXCEPT the MR42 which got a DHCP address was failing to phone home/register to the dashboard. We tried doing a factory reset. No luck. I ran a packet capture and all I could see was arping. No attempts to get out and register. We rebooted everything. I also tried putting the AP onto the wired vlan. But that failed too. Eventually I gave up and just put back their classic Cisco AP which worked right away. I have about 100 branches and Meraki's deployed all over. I've never run into this reluctance to register. Any thought what might have been going on?
Were you able to add the MR42 to your inventory and network? I had a couple rare occurrences a couple years ago where some batches of device serial numbers were not properly populated by Meraki's back end that prevented me from bringing some new gear online. It sounds similar to what you are describing, but maybe not the same.