The 404 page you are seeing is not generated by Netlify.
DNS changes can take up to 48 hours to propagate. See [Support Guide] Why do DNS / SSL changes take up to 48 hours to propagate? (TTL)
That said I see Netlify nameservers
$ dig guesstimateit.com NS
guesstimateit.com. 3600 IN NS dns1.p04.nsone.net.
guesstimateit.com. 3600 IN NS dns2.p04.nsone.net.
guesstimateit.com. 3600 IN NS dns3.p04.nsone.net.
guesstimateit.com. 3600 IN NS dns4.p04.nsone.net.
and associated records and have no issue accessing apex or www subdomain. If you are still having issues, try a different browser, device, and/or network, then wait.