I need help setting up

Nah, that's for your router to translate local domain to IP, basically if you want to refer to other local devices with easily typed domain instead of IP. To use NextDNS, go to Network Setting > Home Networking > LAN Setup, change the DNS Values to Static and enter your DNS Servers from the NextDNS dashboard, also set IPv6 DNS Server to User Defined and enter your IPv6 from the NextDNS dashboard if you have it. For IPv4 DNS to work, configure Dynamic DNS under DNS menu, register your personal account in one of the supported provider, enter your DDNS hostname and credential, then enter the hostname in NextDNS dashboard