Well , long story , I'm fully aware that Meraki products do not fit us at all.
 
Also our 500-600 MX are all in configuration templates so the less we mess with it , the better. 
 
Thanks for the suggestions the public IP is always my last resort ( and I have used it in the past ). 
 
Preconfiguring our MX is always the best idea, but since we are a big company it's not always easy to keep track of everything.
 
I might close this thread since there is no way to configure a mgmt interface. Thanks !