Are you running any firewall on your debian machine? Maybe install ufw firewall and open same ports in there. Then try again.

Forward ports 80 and 443, set them up in nginx config and use certbot to get a SSL certificate if you have a domain for your site.

I think the users need to be in same LAN network. Maybe you can bypass this with setting up a VPN server, but not sure.

Time is money friend.