I'm not sure what you mean with your setup. What is important that you can only use NAT forwarding if the NAT IP's your ISP gives you are in the same subnet as the WAN interface of your MX.  Then you can use port forwarding or 1 to 1 or 1 to many NAT. However if your provider tries to route some separate set of IP's towards your MX that are not in that subnet, that won't work.  Then you need the router in front of your Meraki also doing NAT towards the same subnet.   If you didn't get my explanation let me give some examples.   If the subnet between your provider modem/router is 11.12.13.0/29 and that router IP is 11.12.13.1 and your MX is 11.12.13.2 you can still use 11.12.13.3-6 to NAT towards devices behind your MX.  However if that provider tries to route 11.20.30.0/28 towards your MX at 11.12.13.2 that won't work. ... View more