easiest way is to create default.htm as comming soon page and make sure, that default.htm is listed in the IIS web site configuration, tab Documents above the default.aspx

Thus, users surfing to www.yourdomain.com  will get displayed the comming soon page, while www.yourdomain.com/default.aspx (and all other deep links) will display DNN. If you want to avoid this as well, you need a second (sub)domain, which your are knowing and using exclusively, in this case you can simply redirect the original web site to a different folder while working on it.