How to configure Android to act as server in the local network?

I have two devices (an Android phone and an Orange Pi) connected to the same Wi-Fi network via the notebook's hotspot. I do have a server running on the Android device which I want to make accessible to the Orange Pi. How do I do that? As far as I know Android doesn't have a firewall or anything like that. Accessing the server via IP opens the server OK, but on the Orange Pi, I get that "cannot find host" error and can't ping the Android's IP address. What am I missing? How do I fix that?