for the moment (since ywaers) I have two nqmservers. A primaray ispconfig controlled on VPS Debian 12 with fixed IPv4, and a secondary manuell set up on a debian 13 server here at home, behind a fixed IPv4 address. I do use IPv6. Thats OK so far. Whether MXTOOLBOX nor thr DENIC NAST tool Show errors. As I will give up my fixed IPv4 Adress at home for at least a while in case of moving. I try to add an other secondary at my registrar InternetX. But I allways get back "ERROR: 133 Answer must be authoritative (nameserver, ip, proto, record) (a.ns14.net, 2001:8d8:fe:53:217:160:80:160, udp, NS)" Need help or hint. my domain is muekno.de
Just a guess, since I cannot comprehend your given details that much, and if your setup is meant for being an authoritative public nameserver, in my mind you will need more that fixed IP at home. In @Taleman's signature there're good guides for that.
As checks from MXTOOLBOX and the DENIC NAST report everything OK and the Domain is registered since 1999 and other de domains on these nameservers work fine I assume my setup is OK.
