Hey there! Sorry to hear about the issues you're having with those nameservers. It sounds like things are working well at the registrar level, but there may be a problem on the WHM side. You'll want to make sure the main zone file for domain.com has both the A records for ns1 and ns2 as well as the two NS records for each nameserver. Those should look something like this:
ns1 14400 IN A 1.2.3.4
ns2 14400 IN A 1.2.3.4
domain.com. 86400 IN NS ns1.domain.com.
domain.com. 86400 IN NS ns2.domain.com
Can you make sure all four of those entries are in place?
I just checked that, and the records are correctly set up.
The "bad ping" problem was just the firewall that didn't work as expected and we fixed that.
Now we can ping the nameservers but we noticed that the dns zones of all of the domains under our nameservers aren't correctly propagating.
As you can see from the image, the dns zones are propagated only in France (were our datacenter is).
We can't find a way to make the dns propagate properly.
Also, the main hostname that we use for WHM is under our nameservers. I live near the datacenter and I can access the WHM panel without any problems, but the rest of my team can't access it since they live in other countries.