I solved the issue a while ago and forgot to answer here.
After entering the IP in Captive Portal / Allowed IP Addresses, everything was perfect.
As my CP is authenticated, so I believe that the question was precisely at that point. The other end had no way to authenticate itself to be able to pass and from the moment I released the IP there, he started to communicate. I even thought about doing a test of this type, taking the CP's authentication to see if it worked directly, but I ended up not having time.
Anyway ... it's resolved.
Thanks to everyone who was willing to try to help.