Maybe you should mirror that downstream port to another port on the MS switch. If the moment you connect that Unify and you see many repeated packets you are probably having a loop downstream and need to fix that first. ... View more

This error usually is not lying.  There may be an issue where the interface IP of the Meraki device cannot reach the configured DNS server.  Usually this is an upstream firewall or isp issue.  Or it can be that the normal vlan you configured your devices on is not able to reach dashboard and your Meraki devices are on a different subnet. ... View more

Use the Firewall Logging tool on your MX. https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/Firewall_Logging Monitor port 53.  See if it reports anything being blocked.     ... View more