To solve this issue, on the server side you will need to edit a file in the TSplus installation path.
Go to C:\Program Files (x86)\TSplus\Clients\www\software\html5
Open the settings.js file with a Notepad
1 - How to force the full-screen button in HTML5 mode:
Check that the variable 'W.showfullscreenbutton' contains the 'yes' value:
W.showfullscreenbutton = "yes"; //show fullscreen button in top menu if supported.
2 - How to force full-screen display in HTML5 mode:
Check the variable 'W.full_screen' and change the digit value to '2'.
W.full_screen = 2; //when 0, usual mode, 1 = full screen with fixed task bar, 2 = full screen size, by no full screen support it gets disabled, PC only.
(and to resume normal screen display, use the keyboard 'ESC' key)