Issues with DNS records of external domain Propagation

Hello Support,

My netlify app after configuring the A and Cname dns records of my external domain name to point to my netlify app connects with my domain for some time the custom domain loads and serve my netlify app but after some time it stops and shows just my external domain server default page. Again, sometimes on one Device it connects to my netlify app and other devices keeps showing the external domain server default page. Please help me with this problem.
This is my netlify app:
This is my external domain

While both dig and whois show (for me at least) is using Netlify DNS, I can see on DNS Checker there an non-Netlify nameservers (see such as the three in the screenshot below (there are others.)

Depend on when you changed the nameservers it is possible these locations are slow to update—DNS changes are instant, it takes time for them to propagate.
That said, if the changes aren’t recent, it is possible there is an issue with the registrar and suggest contacting them for assistance.