Your phone needs to be connected to the exact same Wi-Fi network, for example your home Wi-Fi network to be able to open the web interface. Try to find the Wi-Fi client isolation setting in your Wi-Fi router's configuration web interface.
If you want to use the openSPOT on the go with your phone's shared internet connection, then you can find the instructions on how to set up in the user manual at https://manuals.sharkrf.com/openspot4/en/home-and-phone-wifi.html#page-top
