iPhone 8 wifi wont turn on

Wi-Fi and Bluetooth work together in tandem, so as an additional troubleshooting step, please try the following:

Tap Settings > Bluetooth

Is the slider for Bluetooth also greyed out?

If yes, the Wi-Fi / Bluetooth "chip" on the iPhone's main board has failed and will need to be replaced. Make an appointment at the Apple Store or take the phone to an authorized Apple service shop for repairs.

You can find a list of authorized services shops in your area using this link: Find Locations

Then, all that you can do at this point is try a Forced Restart to see if that will reset the Wi-Fi.

Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The Wi-Fi returns to normal operation.....or, if Wi-Fi is still greyed out.....

2) There is a hardware problem with the phone that will still need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations

Then, all that you can do at this point is try a Forced Restart to see if that will reset the Wi-Fi.

Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation.....or, if Wi-Fi is still greyed out.....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations