Why is there no connection to the IP address?

Question

Network routing

I raised the server for development, everything works for two people, one backender cannot enter. I cleaned both the DNS cache on the computer, and the browser cache, and just tried to enter by IP. As a result, I checked the IP address with the ping command - it does not send packets from the backender computer. Everything is fine with me, the server is not on the local network. With what can be connected? On an Ubuntu 20 computer, the DNS entry is old, I created the dev subdomain.

In addition, if you distribute the Internet from your phone, everything works for him. Apparently in the Wi-Fi router, the IP server somehow got into a ban?

A couple of days ago, everything was fine with him, and ssh worked, and he visited the site

Answer 1

The problem was in the DNS of the wifi router

Answer 2

It does not ping the external IP of the server? did I understand correctly? maybe his IP got banned on the server .... if he connects the Internet from a mobile phone and everything starts to work for him ....
well, as a variant, let the router reboot