DVDs were released at different times in different countries or regions. Movie studios wanted to make sure they control those releases and you can’t just easily import and sell a bunch of DVDs from a region with an earlier release date.
Of course there were DVD players that let you switch regions infinitely (and eventually just play all regions) or some people who had a second player for another region, but those were, at least initially, outliers. To make the general public wait to buy DVDs from their region it worked.
Although, one could argue, it gave another advantage to piracy. Not only would a downloaded movie not threaten you with the FBI or make you sit through unskippable ads, they would be available as soon as their release in any one region, so you would have to wait (sometimes a lot) less.
Probably a bit better than your phone, depending on models (phone, extender, router), of course. At least in theory repeaters can have better antennas and more power to spare.
If you can put a network cable though at least one of the walls you might as well just put an outdoor AP (main building) or WiFi bridge (shed) there. That way the signals only have to go through one wall.