Hello again,
You should also check to see that the incoming connections are not blocked by some other firewall, such as a firewall that your host has placed.
To clarify, CoinPayments should only be able to tell you whether the connection was successful (and the message was rejected) or whether the connection was not successful. In the first case it would be a matter of an incorrect IPN secret, while in the second case it would be an issue with some firewall. I am only mentioning firewalls because a number of other users had the same problems with their hosting providers.
kind regards