this works on both single and dual sim and the settings will "stick" after reboot... requires root
https://xdaforums.com/galaxy-s4-sprint/general/how-to-fix-network-signal-gsm-sim-t3720534
https://xdaforums.com/galaxy-s4-sprint/general/how-to-fix-network-signal-gsm-sim-t3720534