One of our Macbooks will not connect to wifi

Several months ago, one of our Macbooks gradually began losing the ability to connect to wifi. It is a MacbookPro 15" and has just been updated to ElCapitan.

The router is not the issue. All our other devices, including the MacbookPro 13" ElCapitan I am using to post this, works fine. We have tried power cycling the router but are unable to make other changes because it is the property of our network provider. Power cycling has had no effect.

We took the problem laptop into our local Mac repair shop and they were unable to find any issues and they updated everything the could to the latest versions. This has had no effect. The laptop will will connect to wifi only if it is directly beside the router. The problem persist with all routers. We have tried using the laptop at friends' homes and again, the laptop will only connect if it is directly beside the router.

We ran diagnostics and prior to ugrading to ElCapitan it had no effect.

After uprading to ElCapitan, using network diagnostics creates a zip file "WirelessDiagnostics_C0rHR599DV35_[date/time]" When we unzip the file it creates a folder on the desktop under the same name which contains a vast number of files and folders. It contains .plist .txt .spx files, system logs, bluetooth files, diagnostic reports, directories, and heaps more information that we don't know what to do with.

We are certain that this is a laptop issue and not a router issue since it is exclusive to the one laptop and happens with all routers, but the Mac repair shop found nothing wrong and we don't know how to use the information we find in the network diagnostic zip files.

What can we do to repair this?