Unreachable just means it cannot communicate to the cloud for some reason. Make sure you have no ACL or firewall rules blocking the management IP address of the switch. Try pinging it from upstream. The fact that your AP downstream can connect to the cloud means it still forwarding traffic, just not able to communicate with the cloud.
You can open a support ticket to have the team look at it and see why its an issue.
Find my post helpful? Please give me a kudo!
CCNP Certified and Meraki Operator