Jitsi Desktop Client using a different SIP port to register

Hellow Everyone!

In order to avoid hacking, we have an SSIP-PBX in the cloud, which does not use port 5060 for mobile and teleworking users to register the softphone. We use a different port, as an example in the registration area of ​​the SIP account we enter the SIP Domain FQDN, Ex: cssip.duck.net:8999.
When public DNS resolves and this request reaches the firewall where the SIP server is located, it changes port 8999 to 5060.

We have tried to register with the following procedure:

3090@cssip.duck.net:8999

Or: 3090@cssip.duck.net and in the Connection TAB, PORT: 8999 (Replacing 5060) but we have not been able to register the Jitsi Desktop client.

We have done this same procedure with other SIP Softphones clients and we have never had problems.

Any idea that we are doing wrong.

We appreciate your kind comments.

Best regards!