We have setup a web server behind a NAT router which we have properly port forwarded the requests to our web server. We have already assigned a dynamic DNS for the WAN IP of the web server (that is the WAN IP of the router).
The problem is: we can access the web server we setup but other clients cannot access to that web server.
We verified that we are accessing the same IP address; therefore I think the problem is not the ISP DNS.
Secondly, we tried curl-ing the webserver, but there is no response in the client's computer.
Thirdly, we tried installing Browsec VPN extension to bypass any potential blocking of the ISP and it worked! Sadly, based on a thread: https://security.stackexchange.com/questions/64123/browsec-vpn-and-anonymizer It shouldn't be trusted for sensitive data.
Lastly, since we have access to the web server but other clients cannot access the web server, we tried following the thread's idea of setting up a reverse proxy web site in our area for them to gain access to the web server but it did not work.
We are already out of ideas as to what is the problem.
We also tried burp-ing the web server from our computer and the client's computer. In the former, after sending the initial GET request, there are many succeeding GET requests that followed while in the latter, there are no succeeding GET requests that followed.
Another observation was, in the client's computer: We gained access to the webserver using mobile data but failed with DSL connection; they are different ISPs.
How do we troubleshoot this problem?