My wifi won’t turn on

You are a patient person, this may be indicative of hardware failure, either with the chipset or antenna. The update doesn’t cause it, it reveals it. Click on the Get Support link at the top right of this page to start the service process. They can test your iPhone remotely assuming cell data is still working.

Try first to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Check the Wi-Fii connection, see the following Apple articles:

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

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

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

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

Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password - Apple Support

If they are On, turn Off Hotspot & VPN.

Update to iOS 12.5.5.?

iPhone 6 is an outdated iPhone (9 september 2014), can’t be update to a higher iOS.

iOS 12.5.5 is last security updates for iPhone 6 -> released 23 september 2021.