No, @Techaddict suggests that the phone does not remember your choice until some reboots. If you have two SIMs, you have to choose the network for each of them individually.
To be honest, I don’t really believe that this is just a matter of rebooting. My last guess would be to go to Engineer Mode and tweek the network related setting. However you would do this on your own responsibility and In cannot lead you on. Maybe they can help you at some Mediate related forum.
