If you change any Domain nameserver, it will require a few hours to reflect globally. This is called DNS Propagation. Generally, it takes 12-24 hours.
Whenever your website accessed through an ISP network, it stores a copy of that Domain DNS. Once the nameserver is change or update, it will take time to wipe out the old copy and store a new copy of DNS resolver. We recommend changing the nameserver while your website has a minimum amount of traffic.
You can check the global propagation from this URL. https://www.whatsmydns.net