if you go to status window and do

/dns host_mask_of_server

and it gives you the ip number of the server, then it's not a problem of your isp's DNS not being able to find it. (though there's a remote chance it's returning an OLD status of the ip number)

if that /dns is unable to find it, then either you've used the wrong server-name, or it's dead-or-down, or maybe the ISP's DNS server doesn't have it.

if you have a friend at another isp who is able to get a reply to the /dns command while you can't, then there's a good chance it's a problem with your ISP's dns.

If your friend gets a reply that the /dns resolves to a number, you might be able to use "/server that-number" to connect.