Active job

How do I locate the studs from outside?

We have a retractable hose reel as shown, that I tried to attach to the cladding as shown. The fixing isn't that good so I have removed it and devised another method to fix it to the wall. The main task here is for someone who knows to come over and show me how to locate the studs from outside. My stud-finder sort of works but I'm not confident, the first result wasn't that great. I'm scared that if I don't attach it properly, it may break and leave a hole in the cladding.