WiFi issues with iPhone iOS update

Without knowing what “tricks” in the book you found, it’s rather hard to help. And Reddit, believe it or not, has very little reliable advice. I’ll make some wild guesses, but don’t bother to tell me the ones you already tried:

• Here is a general answer→If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support. If that doesn’t resolve the problem you will have to provide a lot more information, including, among other things:
• Do you use VPN? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile.
• Have you checked for interference→Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
• Have you tried forgetting the network, then reconnecting?
• Have you tried Reset Network Settings?
• Does it happen with all networks, or one specific network?
• Is it a secured network, or open network?
• What is the message when you try to connect?
• Does the network have MAC filtering enabled (aka Access Control)?
• Do you control the network, or is it someone else’s network? If you control it, restart your router by removing all power for about 15 seconds.
• Do you have Private Address enabled for the network?
• What other troubleshooting have you done?