iPhone 6s Plus cannot connect to Wi-Fi

This does not seem like an iPhone issue. So "Erase all content and settings" may not be a solution. Try the following. --> If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi ...

Consider more secure wifi configuration.

Change your router’s WiFi to use WPA2-AES (CCMP) with no WPA or TKIP. Updating this setting ensures that your devices always connect in one encryption mode

1. TKIP is obsolete and not considered secure. Choose WPA2, the latest encryption standard, with AES encryption for optimal security
2. If your router doesn’t specify TKIP or AES, the WPA2 option probably uses AES

Privacy Settings

1. Toggle off Location Services. Go to Settings > Privacy > Location Services and turn it off
2. Many users find that disabling System Services’ WiFi Networking solves the problem
3. Go to Settings > Privacy > Location Services > System Services > and toggle off WiFi Networking

Try a Reset ALL Settings. This is not Erase

Go to Settings > General > Reset > Reset All Settings

• • This resets to default everything in Settings, including notifications, alerts, brightness, and clock settings like wake up alarms
  • Resetting all settings also reverts all your personalised and customised features like wallpapers and accessibility settings back to factory defaults.
  • Performing this action does not impact any of your personal device data, including photos, texts, documents, and other files
  • You need to reconfigure these settings once your iPhone or iDevice restarts

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support