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How would that work? I'd configure it through my Unifi access point?
Nah, you configure your router to hand out the nextdns address for DNS to all DHCP clients.
IF you can't do this on your Plusnet router, then you'll need to switch it to modem mode and get a separate router. Like a Unifi Security Gateway
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You can set it up on the individual devices but really you want to set it up on your router.
I have a unifi security gateway which it's very easy to set it up on. Lots of guides on the nextdns website. Most ISP routers are rebranded ones from big manufacturers so you may be able to get it working on your Plusnet one.
Personally I'd do away with the Pis and use https://nextdns.io/