Can a RainMaker device work without internet?

Yes, to a limited extent. Local Control lets you control devices on the same Wi‑Fi network without internet access—the phone app discovers devices via mDNS and sends commands over HTTP on the local network. This works after initial provisioning, even when the cloud is unreachable. See How can I implement local control for my devices? for setup.

Schedules also continue to work offline. Once the device has obtained time from SNTP (which requires internet at least once after boot or after a long offline period), scheduled commands run locally on the device. The device uses its internal RTC to fire schedules at the correct times, so schedules keep working even when the device is disconnected from the cloud. For scheduling setup, see How do I implement scheduling in my firmware?.