I happen to run an affordable EU supplier who does DNSSEC, and also AXFR (incoming and outgoing). I offer a free plan from time to time, but not at the moment to preserve resources for paying customer.

https://www.ptrdns.net/

Are you aware that the child zone A(AAAA) records for danube.ns.ptrdns.net differs from the parent zone A(AAA) glue records for danube.ns.ptrdns.net?

Looks like it's the glue records that point to the actual server?