Just to be sure I understand:
When you power the device on, it's taking 30-40 minutes to establish a link with your ISP on its WAN interface?
Have you tried the following? Roughly in order of what I'd do.
1. Changing the cable between your MX and the ISP's device. If cable doesn't change anything, does your ISP device have an additional available port that you could test?
2. Configuring a secondary WAN interface and using that, to see if the behavior changes
3. Connecting a device (usually a laptop with software firewall fully enabled, here) directly to your ISP's device and seeing how long that laptop takes to get a connection, when configured with the settings from your Meraki's WAN? You may need to bounce the ISP device to clear its arp table, or have your ISP clear it for you.
4. Verifying with your ISP that their device does not have a problem of its own?