Hey there,
Connor here from Meraki Support. It's likely your cellular provider is using Carrier Grade NAT.
You'll probably see your IP address of the cellular interface as 10.x.x.x or similar, and if you run a packet capture on the cellular interface you'll not see any VPN traffic, as the provider is blocking this upstream. You'll need to contact them to ask for a real public IP address that is accessible from the internet, else VPN traffic will never reach the MX.
Kind regards,
--
Connor Loughlin
Network Support Engineer
EMEA 🇬🇧
.:|:.:|:. Cisco Meraki