Zigbee tends to do somewhat better than Zwave at passing through water, so that’s actually not that unusual.
Both the Z wave and the Zigbee antennas in the smartthings hub are on
omnidirectional, so they will spread signal in 360° but it starts to spread as it moves away from the hub. So you actually get more spread once you’re out about 10 feet. You see this as an issue in some multi story buildings. A device that is one floor up and immediately over the hub won’t connect but one that is one floor up and over about 15 feet will. It’s weird, it’s just the way the physics works.
OK, as far as getting the Z wave signal into the next room, the easiest way to think about it is like a flashlight. Both rooms are dark, you have a flashlight, and you want to signal somebody one room over. Is there any place in the room where the light might get through?
Typically, you would find airflow around the door, sometimes actually through a in wall receptacle or a light switch box (Although less so with concrete walls), obviously through a glass window… Just is there any place in the room or you think there might be a way to get that tiny bit of light to show through.
I’ve known some people who have replaced an interior garage door with a door that had a tiny glass window in it just to have a place for signal to come through. (Careful on this, though, tinted glass also cuts signal a lot.)
Some people will trim a door a little and put rubber weatherstripping down one side. The signal can usually make it through the weatherstripping even if it can’t through the door.
Sometimes you can drill a conduit, but again, not usually in concrete.
If there’s a second floor and there’s a glass window in the exterior garage door, sometimes you can run signal along the second floor and then down to the garage. That can get tricky though.
Before you start thinking about all this you might want to just take the hub into the main house and test this theory by seeing if you can find the devices at a distance of 10 or 15 feet when you’re not dealing with the concrete wall. Or maybe even just open the interior door, assuming there is one. Just to make sure this is what we’re dealing with. So your idea of putting it in the living room is definitely the right first step.