I have a problem while having 2 SIM cards inserted in my phone. I'll try my best with explaining this.
When I have chosen to use Internet on my 2nd card it works fine, but when I want to use Internet on my 1st card it magically disconnects from cellular connection. Then I have to restart my phone or go to NetMonster App and change network type from automatic to anything else and back to automatic it works. However, when I change Internet connection from 1st card to 2nd card and back to 1st card, it still does not work and I have to repeat that. Weird thing is that when I connect to WiFi I am able to make and receive call via WiFi calling, so the SIM card is definitely being read.
It worked just fine until I started to mess with settings in "both SIM cards active" or when I wanted to turn on VoLTE on 2nd card - via code that I had to put in dialer provided by my operator. I have not noticed that right away, because I was on WiFI when I was doing that.
Both cards work in other phones. Tried other cards from different operators and still the same thing. Sometimes I get 3G connectivity, but Internet does not work and I am not able to make or receive calls. When I switch cards, the card in 1st slot does not work and in the 2nd slot it works just fine. I hope you will help me. Samsung G975F.