I fixed this after resetting both SIMs, manually choosing mobile networks, then switching back to auto choose, rebooting and clearing app cache and system cache AFTER SM-S908U1 froze completely.
After reboot, clearing cache, as phone booted into UI, notification was displayed on main screen saying 'no service'.
After about 35 seconds it appeared that mobile networks were back working.
I then deleted any messages that did not go out, rebooted, and phone is back to 'normal'.
It should be noted that this 'bug' affected Google Messenger also as well as another messaging app I downloaded to experiment with.
IMHO, this is a big 'bug', and affects mobile networks in terms of 'blocking' the send and receive interdependence that extends to RTS.
