The unit goes belly up when sending SMS to a number where the phone has been switched off for several months. Timeout error after 5 minutes and after this no SMS can be sent any number. Only solution is to reboot the unit. I can understand that it times out sending to the number not in use but after this it must be ok to send to other numbers without reboot.
Hello,
From your description likely this could be related with hardware failure. Does this issue occur when trying to send SMS to router from the same sim card you’ve used in the phone?
Have you tried factory resetting the device? If nothing changes I’d suggest returning the device to RMA.