NS records define the authoritative name servers for a domain. They control where resolvers fetch authoritative answers and must match the delegation at the registrar.
If the registrar NS and zone NS differ, resolvers can ignore your zone. Use the NS lookup tool to confirm the published name servers and compare the zone serial with the SOA lookup tool.
Run the NS lookup, then use the DNS propagation check to compare answers across resolvers. If you use DNSSEC, validate it with the DNSSEC lookup tool.
NS works closely with SOA and affects all record types, including A records and MX records.