Just to chime in, make sure you first bring the unit online via a wired internet connection as it will need to download some firmware and configuration.
Once it's up and running the first time, you should be able to use the cellular.
As you rightfully mentioned, in some instances you might want to ask your provider for the APNS and set it accordingly on the Appliance status page.
Please keep in mind that what I post here is my personal knowledge and opinion. Don't take anything I say for the Holy Grail, but try and see!
Appreciate who helps and be respectful of every opinion and every solution offered.
Share the love, especially the Meraki one!