Why the login page doesn't ask to save password on Firefox for Android, Chrome for Android and iOS?

This is a normal behavior as it is a serious security threat if we keep this as an enabled standard browser feature.

What you can do is edit your web portal preference in the Web tab of the AdminTool and add a username / password. 

Doing so, all users will have the same username and password already filled. 

If you want to have a unique web page filled with every users, you will need to rename the index.html file located in C:\Program Files (x86)\TSplus\Clients\www into another name, for example john.html for a login page with john credentials.

You will need to write www.yourdomaine\john.html to access it.