I have a MX64C and while on a wired connection it works fine with auto vpn but as soon as it falls over to cellular it would disconnect. While it was on cellular I noticed traffic on the local SDWAN subnet that the vMX sits on being blocked with a source public IP of the cellular sim and a destination IP which of the vMX local IP so I opened up traffic on all ports within the NSG on that subnet and it connected. For ref I have a NSG on the public IP of the vMX but it's open to any traffic.
I could not find any documentation on this, does anybody know why the behaviour differs on cellular and what ports should be open on the SD-WAN subnet NSG to facilitate autovpn?
