NextDNS uses https://dns.nextdns.io/xxxxx for DoH where xxxxx is a NextDNS profile ID. Period. End of sentence.
But, for whatever reason, Peplink fw 8.2 also requires two IP v4 addresses.
I did an nslookup for dns.nextdns.io against two different DNS providers and got different IPs back.
FYI: these worked. 64.187.227.105 and 155.254.28.98
Every reason to think these would too: 207.246.91.188 and 162.220.223.23
Wonder what happens when NextDNS changes their internal structure and these IPs are no longer valid.