Thanks for sending your site over! It’s not hosted on our CDN so we actually have limited visibility into the auth issues you’re running into. As Shawn, the Netlify CMS lead, says below, hosting the CMS not on Netlify is an advanced use case (still, there may be some useful info in the thread):
That said, what you’re trying to do should be possible! Have you checked out the CMS Slack?
And I’m guessing you’ll stick with your current hosting for your use case, but just in case a smoother CMS setup is the priority, you could use our CDN, so sharing a bit about how it works here: