So here is my issue, I have a website hosted from a virtual machine on my server and am using a dyndns service to point a url to my IP. My ISP recently set up a new modem which unfortunately has its own built in gateway and router. After fighting it to forward port 80 I tested it by trying to navigate to the site via the URL and it didn't work, then I tested it on my phone connected to cell data network and it worked! I am able to visit the site via the URL as long as I am not connected to my network. i find this very weird and cannot figure out why.
I am able to view the site on my network by typing in the local IP of the server.
Any suggestions why this might be occurring?
Yes, this is a pain. Usually your modem won't route traffic from inside that's destined for its public IP address.
When you come from outside, the traffic hits the modem from the external line, and the port forwarding rules get applied, and the traffic reaches your web server. But those port forwarding rules don't get applied to internal traffic. You're trying to browse the web server on the modem, rather than on your server.
I did once find a modem that allowed forwarding of internal traffic, but that was a long time ago, and I haven't see one like it since. What I do these days is to use the internal address when I'm on the internal network, and the external address when I'm not. For things that get scripted, I have a little function that determines whether I'm on my local network or not, and programmatically chooses the right way to address the server.