iPhone 12 Pro Max slowed down and will not connect to WIFI after iOS 15 update

Hi DaimlerAG,

Welcome to the Apple Community.

Apple support have an article online that may help:

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

https://support.apple.com/en-us/HT204051

Wondering how much memory / storage space your iPhone has to operate with.

Sometimes iPhones struggle if there are not enough Gb free for the Operating System.

How to check the storage on your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT201656

Learn more

• To reduce the size of your Photo library, turn on iCloud Photo Library and optimize your device storage.
• Optimize your music and videos storage in Settings > Music > Optimize Storage. 
• Learn how iOS, iPadOS, and macOS report storage capacity.

or

You. might want to check your iPhone's Battery Health & Performance

https://support.apple.com/en-us/HT208387

Hope there is something here that helps you enjoy using your iPhone again :-)

DaimlerAG Said:

"iOS 15 issues: Since I downloaded iOS15 my iPhone 12 Pro MAX has slowed to a crawl, will not connect to WIFI and even when I try to enter my password to apple so that I can do an erase and restore from backup it sends a message back that reads 'Unable to connect to server'."

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Use a Computer and Ethernet:

Start with attempting an update through use of a computer. But this time, user Ethernet.  Go Here: Making Use of a Computer for Updating an iOS Device - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple

WiFi not working after installing iOS 15.0.2 when transferring iPhone 6s to iPhone 8 with Quick Start.

Details:

No WiFi icon on the top of the iPhone screen.

WiFi networks appeared in Settings > Wi-Fi.

VPN app installed.

Solution

WiFi started working after deleting VPN app. 

WiFi working after VPN app installed.