How to properly connect a domain to a dedicated server?

Question

Domain Name System

There is a dedicated server on firsdedic, 2 IPs - 11.222.333.44 and 22.33.444.55.
CentOS 6
ISPmanager Lite 5
Everything was configured while smoking the manual:
ru.ispdoc.com/index.php/%D0%9D%D0%B0%D1%81%D1%82%D...
In the ISP panel under the user, the settings are as follows:
domain.com. A (Internet v4 address) 11.222.333.44
www.domain.com. A (Internet v4 address) 11.222.333.44
ftp.domain.com. A (Internet v4 address) 11.222.333.44
mail.domain.com. A (Internet v4 address) 11.222.333.44
smtp.domain.com. A (Internet v4 address) 11.222.333.44
pop.domain.com. A (Internet v4 address) 11.222.333.44
ns1.domain.com. A (Internet v4 address) 11.222.333.44
ns2.domain.com. A (Internet v4 address) 22.33.444.55
domain.com. MX (mail server) mail.domain.com. priority=10
domain.com. MX (mail server) mail.domain.com. priority=20
domain.com. NS (name server) ns1.domain.com.
domain.com. NS (name server) ns2.domain.com.
domain.com. SOA (initial zone record) root.domain.com. serial=0123456789
domain.com. TXT (text record) v=spf1 ip4:11.222.333.44 a mx ~all
The registrar (naunet) has:
Domain Record type Record
domain.com. IP Address 11.222.333.44 Domain
Names
Domain Record Type Host
www.domain.com. The host name is domain.com.
mail.domain.com. The host name is domain.com.
ftp.domain.com. The host name is domain.com.
DNS servers:
ns1.domain.com 11.222.333.44
ns2.domain.com 22.33.444.55
With these settings, DNS check services complain about the lack of a response from one of the ns servers, the site does not open in about 50% of cases, depending on the provider, the OS of the visitor's device and the desire of the left heel.
Good people, help me get on the right path, where could be the problem?

Answer 1

Logs! See logs... server, DNS, error logs and access logs. Try using free DNS from Yandex to check if everything works - look for a problem in your DNS.

Answer 2

Indeed, why not just register the record type A for Yandex for a start, with the domain name bound to 11.222.333.44, and then try something else? Although, I would stop at this option.