Same netmask?
When asking for network troubleshooting assistance, super useful to provide ANY kind of network info. So far we have WiFi and same subnet. Yet absolutely no details which are necessary to help form questions or provide answer.
Can you post the IP Netmask and Gateway of your Linux server and one of your mobile devices that can’t view the server?
Can you ssh into the Linux server from your mobile device?
Then there has to be a firewall, web server whitelist or some kind of configuration issue with the service being hosted. Because according to all your responses they are on the same WiFi with the same subnet/gateway/netmask.