This domain registrar suggests that if you register a .tn domain (let's say "kit.tn") then you are allowed to set up a server in Tunisia that does nothing but redirects people somewhere else (let's say "kittn.com"). If you do this, kit.tn will be almost unused since Google and other search engines will pick up the redirect and just link everyone to kittn.com. However, if your kit.tn host is unreachable, at least everyone who knew about the .com server can still get to it, which should be just about everyone thanks to Google.
Alternatively you could use a reverse proxy in Tunisia which would appear to be hosting kit.tn, but in reality it is actually sending the requests to another server elsewhere, getting the response from the other server, then forwarding the response back to the client. This could mean doubling your bandwidth usage depending on whether you have the reverse proxy caching pages or not. It would also increase page load times. If your internet connection is good but servers are bad, this may be your best choice. However, if the reverse proxy goes down, nobody would know how to get to your site at all since you were hiding the "real" server.
Personally, if hosting in Tunisia is as bad as you say, then as long as .tn requires local hosting I wouldn't bother with registering a .tn domain. Unless I could get kit.tn...