Unable to join WiFi network on iPhone 6s

Is this just one network, or all networks that you try to connect to? If it's just one network, there may be some security settings or something that aren't compatible with the iPhone, or make sure that you are entering the password correctly for the network. If this is the only device that won't connect to your network, then I would suggest getting in touch with the manufacturer of your router to find out what you can do to get your phone to connect. If it is a router that is provided by your Internet provider, then I would get in touch with them and see if they can assist you with getting your phone to connect.