How to edit main page when redirect enabled?

Hello all. Apologies if this has been covered, but I can’t quite figure out the search terms for my query that will return relevant results.

I am currently working on a site that is hosted through Siteground.com. As the site was already once published, I need to have it redirected to an existing site until we are ready to go live with our site. Unfortunately, this means that whenever I try to edit the main page of the site, I am unable to see the preview, as the preview always redirects to the temporary placeholder.

My site is nunavutparks.com, and I have tried to add /index.php to the address, but that also brings me to the redirected location. I don’t seem to be able to have an option for staging the site prior to uploading. Can anyone offer a suggestion? I will also be contacting Siteground to see if they have suggestions.

Thanks!