Share RPi IP with mobile device to access Web Server

Question

I would like to connect a mobile device to the Raspberry Pi 4 Model B if possible without special software on the mobile device and start a script on the Pi from that device. My approach is to set up a hotspot on the mobile device to connect to the Pi, then use the Python REMote Interface library (Remi) as web-server via the Browser to start the script with a Button. To run the script properly I need an internet-connection.

The Problem: I need the IP-Address of the Pi to connect to the Remi web-server, which changes depending on the mobile device I use. I tried to set a static IP for the Pi, but it doesn't work with different mobile devices.

• Is there any possibility to get the IP of the Pi without using a terminal? -> On Android it is displayed sometimes, but not on iOS.
• Or is there any way I can access the web-server without the IP?
• Or the other way around: Can I set up an access point on the Pi and
  still use the internet of the mobile device, after connecting it to
  the Pi?

Thanks in advance

Answer 1

Or is there any way I can access the web-server without the IP?

Normally (as long as the device trying to access the web-server has Bonjour available), you can access it without knowing the IP address - you just need to know the hostname (e.g. raspberrypi.local would be the web address if the machine's hostname is raspberrypi). I haven't tested this from the device that is actually doing the hotspotting, it might only work for other devices on the hotspot (I was trying to find that out when I found this question)

For your other two points, I could think of particular situations where you could do those, but they don't work in general cases and/or work very efficiently.