Our WebMail also works fine on mobile devices (android, iphone, ipad, tablets, TV box, etc.)
Use the same method to login

• Open a browser
• type sxl.net/app in the address bar
• Click WebMail
• Login with your email address and password
• Save your password on private phone.

Most browser offer an “Add to Home Screen” option in it's menu

• this is an example from Firefox for Android, but other browser have similar function (probably different wording)

After add to home screen, it act like a normal phone app.

• This guy made a video on how to add any website to iPhone home screen: https://youtu.be/vx0IGxVzYh4
• You could use this method to add WebMail to your home screen.