Hi and welcome to Digi Forum.
to better understand the behavior you see, we would need to check logs and status of the device while the issue is happening (so before is manually power cycled).
In order to do that, you should please open a case emailing to firstname.lastname@example.org with the debug.txt file (https://www.digi.com/support/knowledge-base/how-to-extract-the-debug-txt-file-from-a-digi-tran) collected locally while the issue is happening.
Please also provide the IMEI of the affected router so we can do a warranty check and proceed with Base support.
More details about Digi support options on our website: https://www.digi.com/support
Also, you may want to have a look at this guide: http://ftp1.digi.com/support/documentation/AN07_TPort_Cell_Prob_Det_and_Rec_using_SureLinkWiz.pdf that provide instructions on how to configure SureLink in order to detect and recover cellular issue automatically.
Digi Technical Support Team