How to show the desktop version of the site on the mobile version?

Question

HTML

Good afternoon. A very interesting question arose. There is a site adapted for desktop (1200) and tablet (768), on mobile (320) there is a stub. The viewport meta tag is standard:

<meta name="viewport" content="width=device-width, initial-scale=1">

The client now wants to show on the mobile version of the site the non-responsive version (i.e. desktop to scale) while keeping the tablet version as it is.
After a little stubbornness, I realized that the task is not the most trivial. Maybe there are some solutions?

Answer 1

What does it mean at 320 there is a stub? The server that gives for mobile phones?