Hi @Bryan_Trang, sorry to hear about the trouble, could you help to share the site domain? If you want to share that privately, send the info to the success team from our contact page here: https://university.webflow.com/contact.
The www domain should be set as the default domain, that should solve the redirect problem if both domain DNS settings are updated according to the instructions: https://university.webflow.com/article/how-to-enable-ssl-on-webflow-sites
In some cases we may need to manually apply the SSL to the root domain, so if the www domains works and the root domain url does not, then contact to our success team for help with that.
Aside from that, I do not know of a reason that register.com should not work, although each registrar/dns provider may be a little different in how those settings are made. Screenshots of our dns settings in Register.com will help, those can be sent via the contact form linked above.