First of all check /etc/resolv.conf and make sure that your nameserver is the first in the list ,or use "dig mydomain.com @ns1.mydomain.com" to use your dns for your queries.
Without config files it's difficult to try to find, what the problem is.
Note also that the following dig output (nxdomain):
dig -x 18.104.22.168
; <<>> DiG 9.6.1b1 <<>> -x 22.214.171.124
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 62017
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;126.96.36.199.in-addr.arpa. IN PTR
;; AUTHORITY SECTION:
173.78.202.in-addr.arpa. 8899 IN SOA ns1.hellorainbow.com. postmaster.hellorainbow.com. 1108639242 10800 3600 604800 38400
means that most likely you're not authoritative to the reverse zone, unless you own hellorainbow.com. You must ask your ISP to delegate you the zone for the subnet you own.