Checked with support and they promise you’re not blocked and your IP is whitelisted. Unfortunately I don’t know how to solve this at this time. I can provide refunds if needed. In the mean time will you privately reply which host you are hosting with so I can mark a note and possibly perform further investigations?
We rechecked it, the mod_security is not enabled on the server.
Probably there are some another dependencies exist which can influent to remote access to https://www.inboundnow.com/pro-api/key/check/ ?
As we still have the 403 error when try connecting to the remote server from wp admin panel.