Working fine from my end. Nameservers good. DNSSEC not used.
www redirects to the shop which is ok.
However, for gflai.com.vn it looks like you’re missing an A record
See below article how to add the @ record so your naked-domain would be accessible too for the users who are trying to access it without www prefix:
Same thing as above written, www working, but missing record for naked-domain gflai.vn.
It might in case where, it could be something from your location or related to the ISP. I remember there were lately topics where sometimes local ISPs have had trouble opening a Cloudflare Website where the IP starts with 188.x (www.gflai.com.vn)→ from Egypt or some other country, as I remember