Not sure if this thread can help also [How-To] Situations App but I can try to describe breafly. And FYI, you want to make sure that the app is opened after every reboot etc. since it can’t start those background processes automatically.
So first of all, in Situations app you’ll need to have some features / properties (not sure what the navigation title is in English) downloaded and installed. These features are screen, mobile data / network and you might also want to have the phone call status.
Here I have my 4G mode when the screen is on. There’s also the check for “Network on” but that was only for debugging reasons when I had some issues so it’s not necessary.
As you can see, 3G rule is set when screen is off.
As you might have spotted there’s the rule for not having a phone call since I experienced some issues with phone calls in general when it changed the network during a call. Not sure if it was related to that but no issues any longer with those rules.