Why my iPhone won’t connect to Wi-Fi connected to other devices

If Wi-Fi will not connect 

·      Reset the router to factory settings (menu or paper clip) / only if you are authorised by the home owner

·      On iphone Settings, General, Transfer iphone or reset, reset, reset network settings

·      If you have a VPN you are strongly recommended to uninstall it and its profile in Settings, General, VPN and device management 

Here's some more steps you can try for Wi-Fi connection issues that may be helpful if your phone will not connect to WiFi

If your iPhone, iPad or iPod touch won't connect to a Wi-Fi network – Apple Support (UK)

eileen ~ Does your iPhone 6s Plus still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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Now solved i see - out of interest how did the battery Heath fix your wi-fi?