The router settings should be protected by a strong password. That prevents others from changing any settings, including DNS. Even if the DNS setting was locked in the router, that wouldn't prevent others from changing the DNS settings on their devices or using a VPN to obscure traffic and bypass router settings.
If this is for use with a pi-hole, the whole point of the pi-hole is that it replaces the DNS server settings on the router. The pi-hole can reference any external DNS server. A good choice these days is 126.96.36.199 since it is fast and claims to not track DNS requests. DNSBench by GRC can be used to find the fastest servers to use. If the pi-hole is configured to use DHCP, it will probably default the the DNS server used by the router which is usually the ISP's DNS server. It's not a good idea to use that since it raises some privacy issues. If privacy is a concern, there are some DNS servers that disable reverse lookup and do not track users. Some VPNs provide their own DNS servers and do not track DNS.
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