If you have terminal access (and a sudo-enabled account) to your server then try typing the following to see if you have a firewall active and what ports are allowed:

sudo ufw show added

if port 80 is open you should see an entry like this:

ufw allow 80

If not then you need to open port 80 by using the following command:

sudo ufw allow http

I would add a few other ports too (for https and SSH)

sudo ufw allow ssh
sudo ufw allow 443/tcp

Hope that helps