Are you rebooting the upstream gear? DHCP or static IP? I suppose you tried configuring a laptop and connecting to the circuit directly? You can try paperclip reset on the MX too.
From: https://documentation.meraki.com/MX/NAT_and_Port_Forwarding/Troubleshooting_Port_Forwarding_and_NAT_...
If the MX was recently installed, or its WAN IP moved over from another device, the upstream device may still be using the MAC address of the pre-existing device. Ensure that traffic from the upstream device to the MX is being sent to the correct MAC address. This can be resolved by rebooting the upstream modem/device or forcing it to clear its ARP cache. For more information, review the article on 1:1 NAT rules not working properly after installing MX. The diagram below illustrates traffic being sent to the wrong MAC address, and thus not being translated by the MX to the LAN side.
- Ex community all-star (⌐⊙_⊙)