It's not possible to use a WiFi equipped MX as a gateway for a Meraki mesh, so probably better you change your chosen Solution reply...
For clarity, this is from the Repeaters setion of the Mesh documentation: https://documentation.meraki.com/MR/Wi-Fi_Basics_and_Best_Practices/Wireless_Mesh_Networking
"Note: Only Cisco Meraki access points can function as repeaters and gateways. Wireless MX security appliances, Z-Series teleworker gateways, and third-party access points cannot participate in a wireless mesh."
I'd recommend you use a non-WiFi MX and connect your didicated repeater MR (or CWxxxx-M) access point on the LAN side of the MX - then all should be good, provided you have decent wireless connectivity between your repeater and your mesh APs and you don't over-subscribe the inter-AP channels too much.