Iphone turns off Wifi when a file is uploaded to a site on the local network. Why? How to fix?

Question

iPhone

Good afternoon)
There is the following system
Laptop (with Denver) - router (mikrotik) - cell phones connected via wifi
There is no Internet in this system.
Phones connect to wifi and open the site (from a laptop) by ip, go through the pages, send js requests to the server and everything is fine.
The site implements the ability to upload pictures from the phone through the standard "form"
Android devices easily open their "camera or documents" dialog boxes, allow you to select a file and upload it to the server perfectly.
And with the Iphone, problems begin here - when you click on the "select file" button, the "camera or documents" dialog opens, and when you select any item, the Wifi connection is instantly disconnected.
Question 1
How to overcome it? Preferably at the site or router level, not a specific phone.
One of the versions of what is happening is that the iphone "does not see" the Internet in this Wifi network -> hence the question number two:
Question 2
How does the iphone determine the presence of the Internet - where does it go and how?
And is it possible to somehow deceive him and make him think that there is Internet in that Wifi network?

Answer 1

What helped me!
I just deceive the Iphone, as if there is an Internet on the network (I fake its requests for Success).
And that's it, the iPhones immediately stopped disconnecting from the network, and the Internet never appeared in it.
Thanks to everyone for the advice, putting everything together made a simple and working method

Answer 2

Are you sure that the problem is not with a specific device?
Look for information about the capative portal settings, the topic is similar.

Answer 3

There, in the iPhone settings, there was a function that allowed using the network if the Wi-Fi was slow. maybe it's about her.