Dynamic DNS (DDNS) via Mobile App
Allow the mobile app to update the “Linked IP” for a profile in the background making it seamless for the user.
You didn't understand, "Linked IP" is only needed for ipv4, which the app does not use. If you use a DDNS as I do (my isp is offering this service and I have dynamic IP and the router is from my ISP and I can use only the ipv4) : "You are currently using xxxxx.xx.xx as DDNS hostname, your IP will be automatically updated every minute."
Anyway, use ipv4 or ipv6 only if you can't use DNS-over-Https or DNS-over-TLS/QUIC, see here: https://help.nextdns.io/t/m1hmv0k/which-setup-type-to-use
I recommend you to read the documentation from Knowledge Base.
And when you refresh "Linked IP" it must be done from a device with the "correct" ip, if on the phone you use a VPN or the data connection it will be a different, so wrong, ip.
I use different profiles for different devices. For me NextDNS is amazing once I got it how to use is it, and I use it since the beta.
What service set identifier (SSID) has to do with NextDNS?
If you use the NextDNS app, another app for DNS or if you set private DNS in phone and you set it with DNS over https or DNS over tls it does not matter what the wifi network support or is set to.
There isn't anything more seamless since the NextDNS app once you put your profile will use it connected not with DNS v4.
On my router I disabled ipv6 and the DNS v4 is set to use AdGuard DNS. Yet, on my TV I set one of my profiles from NextDNS. On my phone I use AdGuard app and I set it to use NextDNS. Even if the network is only ipv4, https and tls work. As I said, what the router is set up does not matter, each device can use different DNS as long as they have the option to change it manually. The DNS from the router is only used if the device connecting to the router is set in auto.
As I have it now, using AdGuard app, I can connect to any network and I will always use NextDNS, no matter how that network, router, is configured, which DNS is set up.
Since my phone is set in AdGuard to use DNS over https or tls I never need to update my Linked ip.