Not all leaves are turning brown. I've had them for about 2 months. It was full of blooms and the leaves were green, then flowers died off and the leaves started turning brown.
Rose Tree Leaves Turning Brown
First please take a look at my article on Tree Roses to understand that they are "built" by grafting. Here is a link: https://www.gardeningknowhow.com/ornamental/flowers/roses/learn-more-about-tree-roses.htm
Then investigate the entire area from the graft of the rosebushes onto the "trunk" of the tree roses all the way down to the roots. See if there are any bored holes in them anywhere as borers sometimes cause problems like this. If it has been extremely hot, temps in the 100's or high 90's for several days, it could be that they are not getting enough water. It could also be that they somehow got sprayed by wind drifted weed spray. If they are still in the pots you bought them in, it could be they are becoming root bound too and need to be transplanted into your gardens or bigger containers/pots.
Here is an article that may be of help to you as well: https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm