Wifi problem iPhone 6plus

Start off by power cycling your router. Unplug it for about 15 seconds, then plug it back in and wait for it to cycle on, then try to connect your wi-fi. See if that helps. If not, then on the iPhone, go to Settings>General>Reset>Reset Network settings. This will erase all of your network passwords, but then go into Settings>wi-fi, select your wi-fi network, and then connect.

It sounds like that is some type of protected network that is requesting password. If your device is not actively using the internet, wi-fi goes to sleep, and that could be disconnecting you from the network. You would have to check with the owner of that wi-fi network to see what their settings are.

Tried what? You have the option to make a Genius Bar appointment at the nearest Apple store and have the device examined by Apple. They will explain your options from there.