Q: Wifi connections on iphone 6plus

Take a look at this Apple Support article:

https://support.apple.com/en-us/HT204051

Reset your network settings.

Settings - General - Reset - Reset Network Settings

Reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

If this does not help, restore your device to factory settings. Note that this will erase the data from your device.

Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.

If it does not help take your device to the Apple Store for further diagnostics.