iOS 26.4 causing serious issues (battery drain, Wi-Fi problems, overheating)

You aren't speaking with Apple here. This is a user to user only forum, which Apple neither participates, nor reads for user feedback.

While I have no doubt you are having issues, it's not likely caused by iOS 26. There are millions upon millions of people out there with iOS 26 who aren't having any issues. Like me, for example. All of our devices are updated to iOS 26, Apple WatchOS 26, iPadOS 26, MacOS 26, with zero issues.

What we need to figure out is why you are having issues.

Things which can cause issues like you're having could be a failing battery or lack of free memory. You didn't tell us what model iPhone you own, but if it's more than 2 years or so old, the battery could be failing. Open Settings > Battery > Battery Health - what is the current health listed there? If 80% or less, your battery is failing and needs to be replaced at your Apple Store Genius Bar.

Now for storage, open Settings > General > iPhone Storage - how much space do you have left on the phone. If you have less than 12GB or even lower, you need to delete things on your phone to free up space, such as Photos, Videos, Music, Apps, Messages, Email, etc.

Do you have a VPN installed on your iPhone? VPNs cause all sorts of connectivity issues and they aren't needed unless one is required by your school or employer. If you have a VPN on your phone, remove the VPN profile and any associated Apps, then follow the procedure I'm including a link to below.

Finally, after you updated your phone, did you or do you ever Force Restart it? I Force Restart after every iOS update and this procedure is recommended when you experience issues --> Force restart iPhone - Apple Support

"Yes, I have already submitted an official complaint to Apple and also had my device checked at a service center. I did not experience these issues on iOS 18, but they started after updating to iOS 26. The service center also mentioned that the problem is likely related to the update."

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Overheating After an Update:

IF App insists that it will be fixed in "future updates", then you may want to wait it out. In regard to the grey screen, there any bulging in the iPhone? If so, then that would indicate that the battery is corroding, and you should get this serviced, as it has become a Health Hazard.

Clear the System Data:

System Data is caches that are created by applications for embrace of usages, so that they can open quicker, and operate for sufficiently. None of this is needed. So, see about creating it all. Many out there don't realize it, but it can take up extensive amounts of storage. Use my User Tip: How to Clear "System Data" in your iPhone's Storage: - User Tip

I installed it yesterday and it suddenly was overheating and it could not be used for some time, a popup showed up telling me that. The phone was really hot. Battery drain was extraordinarily high, battery was dead after 3,5h after using HERE navigation system and a couple of calls

Great news, I hope it arrives quickly