Why Linked IP

Since every NextDNS user can have their own multiple configs, the NextDNS server needs to be able to tell what configs they're trying to use.

Notice that your NextDNS config IPv6 address starts with 2a07:a8c0 and 2a07:a8c1 followed by your config ID.  Due to the massive IPv6 address range, NextDNS can give two (or even more if they ever want to) IPv6 addresses for each config.

With DoT and DoH, the config ID is passed through SNI (for DoT) or through the URL (for DoH).

There's no such luxury with the plain unencrypted IPv4, NextDNS only owns 9 /24 subnet which translates to 2286 IPv4 addresses, not enough to link with every config uniquely. And thus the IP linking, where you tell NextDNS "if the query comes from this IP, assume it's for config X". They do allocate several IPs for it so you can use multiple configs from the same home (ie, your home IP can be linked against different NextDNS IP, which translates to different configs).

Is there a reason you are unable to just run the NextDNS CLI on your router (preferable) or on each device you own? There are a number of advantages to this.