I am also facing the same issue using Xbee3 NB-IoT module.
Where it stops working after an arbitrary amount of time, which varies between 2mins to 17hours.
I am using deep sleep in between the transmissions.
Even though I have programmed in a machine.soft_reset() in the code in case it fails to connect to the network/internet after 30 tries(2seconds interval), it does not seem to work. That means even if it is rebooting, it is not getting connected to the internet/network.
If anyone found any solution please do share.