It is possible for QR to retain the remaining three legs of your booking, but it won't. You'll be considered a no-show on the TUN-DOH leg and the rest will auto-cancel and you'll have to buy a new ticket at walk-up prices. There is a very slim chance (depending on fare terms) that an agent will be willing to reprice your ticket starting from DOH, but I suspect that if they do, this will be at a substantially higher price.

Your best bet is to get a much earlier flight into TUN.