I had been struggling with this for a long time, but finally it's working. I have very little knowledge about networking but was able to set up the P3 and access the webserver locally, but not from outside home. The weird thing is that I also set up an IP camera that I could access locally and over the internet.

After days of googling and searching the web, and lots of trials and errors, I found out that my ISP blocks many ports, including port 80. So after some trial and error, I set up the webserver on port 443 and, hurray, it's accessible from the internet (need to add :443 after the address).

I am just making this post so that anyone with a similar problem may find some help. It's been a learning exercise for me, and a very good one.