Strange behaviour when loading our WPMU website without www.

When I try to load our website with (www.) before the domain name the home page of our website loads normally, but when I try to remove the (www.) form the URL for example write http://domain-name.tld I always find myself routed to the signup page automatically!!

I've tried searching to see if someone else is facing the same problem but couldn't find anything

So can someone please tell me what I might be doing wrong here?

Thanks in advance for your time