Issue with Android devices refuses to use Wi-Fi local network

I have the following situation - An android device and a laptop should connect to the same network in order to allow me to stream some data from phone to laptop using RTMP.

The issue is as follows - when the router is connected to internet (ethernet cable) - everything works. When the cable is unplugged, no global internet connection exists - I can't start my RTMP stream. This is planned for mobile use, so I won't be connected to the internet.

After a bit of digging I have found fakeinternet service, which I have installed. While this works to trick my Windows into thinking there is an internet connection, my android just goes from "No connection" to "limited connection", and refuses to transmit data.

Any advice on how to configure the router or the devices to allow for connection? In short, I just need the phone to connect to the computer via the router - when no internet connection exists..

OpenWRT version is 22.03.2.

Which Android version is that? If you can sniff the packets (using wireshark for example) your Android uses to determine limited connection I can try to add the appropriate replies to the fakeinternet package.