Bird with broken leg ( dunnock I think )

so just today my sister found a bird with a broken leg outside. We think it fell from a tree and it kept moving around but not on its legs, it kind of rolled around. We would have taken it in but we have a cat and don’t want the bird to get more injured. First we made a box with holes so it could breathe but it was moving so much and we put it back in its original spot under some plants . Right now it’s just under a upside down plant pot and has holes for air to go through but we don’t know what to do next( we are not sure if it’s a dunnock but it looks like one and we haven’t had time to look at him properly as it keeps moving)  if anyone knows what to do please tell us

many thanks