iPhone 11 with water damage after 5min under rain

IP68 is a laboratory test under controlled conditions only.

If your phone is now damaged the only option is to pay for an out of warranty repair, which if you do not have AppleCare+, will be $399.

• Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support
• iPhone Repair - Official Apple Support

Because certification tests for Watches, phones, portable electronics, etc. are done under carefully controlled laboratory conditions. The water temperature is tepid and they don't drop the device in the water it is carefully placed there so there is no water pressure. And the water resistance diminishes over time. Most importantly Apple (and almost everyone else) does not provide warranty coverage for IP68 rated water damage. Rain has water pressure and your case holds the water in.

Coming from the watch world I would never get an IP68 rated device and use it in the rain or around water. Your only options are to do an out-of-warranty replacement of your iPhone (this is a assuming you do not have AppleCare +), recycle it and buy a new phone, or find a reputable 3rd party repair site that has experience working on water-damaged small electronics. If you go the latter route Apple may refuse to work on your iPhone in the future.