Troubleshooting

Test DNS TCP

$ dig +short melbourneit.net. NS
ns4.melbourneit.net.
ns1.melbourneit.net.
ns3.melbourneit.net.
ns2.melbourneit.net.

$ (for NS in $(dig +short melbourneit.net. NS | sort); do dig +noall +answer +nottl "$NS" A "$NS" AAAA | sort; done)
ns1.melbourneit.net.    IN      A       203.55.142.15
ns2.melbourneit.net.    IN      A       203.55.143.15
ns3.melbourneit.net.    IN      A       203.55.142.16
ns4.melbourneit.net.    IN      A       203.55.143.16

$ nc -vz 203.55.142.15 53
Connection to 203.55.142.15 53 port [tcp/domain] succeeded!

$ nc -vz 203.55.142.16 53
Connection to 203.55.142.16 53 port [tcp/domain] succeeded!

$ (for IP in 203.55.142.15 203.55.142.16; do dig @"$IP" +tcp +noall +answer +norecurse +nomultiline +nosplit melbourneit.net. SOA | sed -e 's/$/; '"$IP"'/'; done)
melbourneit.net.        3600    IN      SOA     ns1.melbourneit.net. admin.melbourneit.net. 2023050400 86400 7200 3600000 172800; 203.55.142.15
melbourneit.net.        3600    IN      SOA     ns1.melbourneit.net. admin.melbourneit.net. 2023050400 86400 7200 3600000 172800; 203.55.142.16

To get a servfail, run

dig @1.1.1.1 dnssec-failed.org