how do I fix a 504 gateway timeout

Question

I have a very simple hello world script in my /var/www. I tested it out by entering my internal ip address into the web browser and it worked.

I then forwarded port 80 and tested it with my external ip address (but still from my home network) and it worked. However, as soon as I leave home and try to access it from anywhere else I get Error 504 Gateway Timeout.

I tried with both apache2 and also a web server program called shiny and get the same result every time. Any googling I've done says in order to fix this email the webmaster, but I'm the webmaster and don't have know how.

My other servers (ssh, RStudio, games, etc) all work fine from outside my home.

edit:

The gateway timeout error is only from my phone. When I try from my laptop I get

Oops! Google Chrome could not connect to ...

where ... is my public ip address.

Solved (kinda):

Using a different external port seems to work. My best guess is that my ISP has port 80 blocked. This solution is acceptable for me because I'll be the only one using the webserver,but if I needed to host a public website from home, I guess I'd need to call my ISP.