When a scheduleOSUpdate command is issued to an iOS device on LTE, the device stops communicating with the MDM server after processing that command. All commands issued moving forward remain in the queue indefinitely. Communication is restored as soon as the device is put on Wi-Fi.
We have informed Apple of the issue and this article will be updated once we confirm a resolution in iOS. If you have Apple Care Support you may wish to reach out directly to Apple to learn more details around the status of this issue.