Hi. So, the first thing to check is if the policy was applied correctly. You can do this in dashboard by looking at a device, and scrolling to certificates:
So, that's the first place. The second place is to check the installed certs on the device, which you've already done.
The third place is to go to Settings > general > VPN and Device Management > meraki Systems Manager
You'll be able to see everything installed:
If you click on More details and scroll down to certificates:
You'll be able to see which certs have been installed through MDM
Now, if you see your cert here and NOT in the certificates list of the device, it may just be that iOS has chosen not to show that cert
I had a look at : https://developer.apple.com/documentation/devicemanagement/certificateroot and there's nothing to suggest that iOS requires a particular type of cert when being deployed with MDM
Let me know how you get on,
Kind regards,
P