Hi Timelapse,
As a first step and following best practices, please update your device's firmware to the latest version: "RUT2_R_00.07.02." I will drop the download link below:
https://wiki.teltonika-networks.com/view/RUT240_Firmware_Downloads
Then, check if the issue no longer persists; if it does, verify you have data on your mobile SIM card. You can insert the SIM card on your mobile cellphone and check if you have mobile service and data connection. Also, it would be good to check if there is an SMS message from your mobile operator informing you your service is running out of data, reaching a determined data limit, or any outstanding invoices.
Furthermore, it would be helpful to know if you have recently done any configuration that could forbid internet access to your device.
Lastly, please confirm if all the devices presenting this issue are in the same mobile coverage area or different coverage zones since the event could be related to the connected cell site.
I look forward to reading your comments.