Downgrading of any version of iOS isn't supported by Apple. If you search the web, you'll find instructions that may work, but you're on your own should any problems arise. For many people it goes without problems, but for some it's reportedly bricked their devices.
Perhaps if you post details or do a search of the forum, someone can help you sort out the WiFi problem without restoring to hacks to try and downgrade. Meawhile I presume you've tried the steps in this support article?