I am also facing same issue. First sim works fine but 2nd sim shows no network most of the times even when i am in full network. Until i reboot or disable and re-enable 2nd sim, i don't get network on 2nd sim.
Sim 1: Vodafone VoLTE
Sim 2: Airtel VoLTE
also device is already updated to Aug 18 security release. This is really annoying issue.
I am facing tge same problem after the august security update.......my sim1(vodafone) doesnot show any 4g network and my sim2(airtel) shows a 4g network but its too slow...........moderator please read these comments and investigate.......inform tge higher authorities.......there might be a problem in the august security patch