It sounds like the Dashboard has not collected enough cellular data information to plot the usage from device yet.
https://documentation.meraki.com/SM/Monitoring_and_Reporting/Cellular_Data_Management_with_Systems_M...
"On iOS devices, the Systems Manager app needs to be running in the background to update Dashboard and report the latest cell data usage. The usage reported by SM may also vary from your service provider's official reports, as some providers do not count traffic to specific apps or websites against monthly limits."
To fix: try to install the SM app, launch it at least once, and then keep the Settings > Meraki MDM > Background App Refresh enabled (default setting). After that give it a few hours and the Dashboard will likely have enough information to start plotting data usage.
