iPhone Connection problems to web server on local network

We are currently on a ship, the ship has a web server to give details of the ship's services.

On ios 15 passengers have to turn Wi-Fi on and mobile data off.

Sometimes the iPhone won't connect to the webserver. Even using the IP address of the webserver does work.

Restarting the phone does work.

Has anyone any thoughts on what happening and a possible solution?

Thanks