May be related to the 4G/LTE data routing capabilities of the nearest cell tower for your cellular service provider that you routinely connect to.
Take the mobile devices on a road trip, far enough away from where you're currently experiencing this issue that your cellular data is connecting to a different cell tower. Does the issue continue?
If the network routing error exists at a level beyond the local nodes, you may have to travel a much further distance to either find relief from a different network pathway, or confirming the error still exists and may somehow reside within your device itself.
Have you taken the devices on a trip for work to see if the error continues even in the outstation cities hundreds of miles away from home?