The first step to debug this would be to log on to coinpayments.net and go to IPN history.
See if the messages are delivered. They are probably not being delivered.
Click on the date/time of a failed IPN message and see the details. There should be some information there as to why the messages are not being delivered.
Besides wordfence, there can be other reasons why the incoming messages are not reaching your site. Possibly some firewall.
Please let me know what you found in your IPN message history.