In addition to @cmr said.
Here are the steps to set a static route on your MX:
Navigate to Security & SD-WAN in the Meraki dashboard.
Select Addressing & VLANs under the Configure tab.
Scroll down to the Routing section.
Click on Add a static route.
Enter the Name, Subnet, and Next hop IP.
The Subnet is the network you want to route. The Next hop IP is the IP address of your Layer 3 device. This will allow the MX to route traffic to the Layer 3 device.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.