Meraki MX devices do not have a VDSL interface. So I'm guessing you have something bridging VDSL to Ethernet (otherwise you will have double NAT happening). If this is the case, you should configure the MX to use normal DHCP.
It is possible your service provider could be using a VLAN tag, and depending on your VDSL bridge configuration, you may also need to configure a VLAN tag on the MX.