Debian VM, firewall blocking external PHP accessing NodeRed App on port 30300, Android App works OK

[pctborges]

I have a Node-Red App that works as an API responding to http requests on port 30300.

The App URL is: http://alprbr.com:30300/alprbr.

External App submits (using http POST) some parameters and, in return, should get a JSON response.

Android App running on my smartphone works OK.

Some other server running PHP submits http POST to Node-Red on port 30300 and gets no response.

Node-Red console reports no access at all.

Installed PHP App on the same server as Node-Red (Debian VM) and changed target server from external address to http://127.0.0.1:30300/alprbr  and PHP gets the JSON response successfully.

I also have another APP developed on C++ running in a microcontroller and accessing the same API and it works OK as well.

What might I be doing wrong?

Thanks

Paulo