Looks like you can now get and set SIM / APN info. However my testing found it only works with MG41E, MG21E come back with an error that says it not supported.
https://developer.cisco.com/meraki/api-v1/#!get-device-cellular-sims
https://developer.cisco.com/meraki/api-v1/#!update-device-cellular-sims
meraki.exceptions.APIError: devices, getDeviceCellularSims - 400 Bad Request, {'errors': ["This device's firmware does not support SIM configurations."]}
Above error is on an MG21E thats has the lastest firmware (1.9)
The docs made no mention of this limitation.