Laravel ,font-family works on Ubuntu but not on Debian

I have an Problem.

I have coded an Laravel 5 Script on Ubuntu 17 and want use it now on Debian.

But the font font-family is not the same and it looks not good.

On Ubuntu i have installed on my Machine all Fonts i need and add it in my Body CSS:

body {
  font-family: "Roboto Mono", Helvetica, Arial;

Now i have installed also on Debian the Font files for the System and restart the Browser but the Laravel script dont use it, why? Or have i forget something?

Thanks

via Chebli Mohamed