What is Linked IP
In order to associate a configuration with a network or a device, NextDNS uses different tricks depending on the DNS protocol used. For DNS over TLS (DoT), the configuration id is embedded into the hostname, for DNS over HTTPS (DoH), it is in the URL path and for UDP over IPv6, it is in the last bits of the IP. Unfortunately, for legacy UDP over IPv4, there is no such easy trick. Because IPv4 is a scarce resource, it would not be possible to attribute 2 IPv4 to every configuration. Instead, you have to associate the IP of your network with your configuration. We use a pool of IPv4 to let you link the same IP with different configurations if needed. You only need to link your IP when using unencrypted DNS over UDP/IPv4.
To link your IP, you just need to click on the link button in the setup tab of the interface. You will have to repeat this operation anytime your IP changes if it is dynamic. You can also opt for our DDNS solution: Instead of manually linking your IP, you give us a hostname that points to your IP, and use a DDNS solution to keep it up to date. The third option is to call a URL from a script whenever your IP changes.
With DDNS linking it says it gets updated every minute, AFAIK most ISP's assign new dynamic IP's anywhere between 1-3 days & thus checking the IP every minute seems unnecessary use of your resources IMO. Could make it longer in the scale of hours maybe.
This is just my thoughts, I am a novice so there might be a very valid reason to be checking the user IP every minute.