I have no more internet on my iPhone since the iOS 15 update. What do I do?

Unfortunately, this update can indeed cause issues. Can you try the following?
Go to Settings -> Mobile data network -> Reset.

If that does not help, you can try resetting your network settings via Settings -> General -> Reset -> Reset network settings. Unfortunately, you will need to add your Wi-Fi passwords again afterward, as well as re-establishing your Bluetooth connections. 

If it still does not work, can you enter and save the following settings under Mobile data network?

APN: web.be
Username: web
Password: web

You may need to reboot your device afterward, but that should do the trick!