After updating to 17.3.1. Safari won't work

Hello Ewan Lamont, On your iPhone in Settings you might check this setting. Settings > Your AppleID name > iCloud> Scroll down to iCloud+ Private Relay. If it is ON try turning OFF and a reboot. If it is already Off then not sure. Let me know.

Sorry to hear that you’re having issues with Safari on your iPhone, you may find this Apple Support article useful: If Safari isn't loading websites or quits on your iPhone, iPad, or iPod touch - Apple Support

You may want to try force restarting your iPhone: Force restart iPhone - Apple Support

If you’re still having problems after that, you should contact Apple Support: https://support.apple.com/iphone

Tried all suggestions and iPhone 13Pro Safari still will switch to 401 error code “url not found” on every website. Turned off vpn went to phone data. Pretty frustrating. Started after 17.3.1 update

I had to go back to an old phone as my phone stopped working. no one could fix it.