How to understand why packets do not reach?

Question

Apache HTTP Server

Installed LAMP. From the local network, I can open the site on the external (WAN) IP of the router, the default Apache site opens. When accessing the same IP address from the Internet, the browser gives a "Page not found" error. On the router, the ports are forwarded, the DMZ is also enabled on the server. SSH, WoL and the rest work without problems.
How to determine if the problem is that the router processes packets coming from the LAN and WAN differently, or the packets simply do not reach the router? Router: Eltex NTU-RG-1402G-W

Answer 1

To determine whether packets reach the router, you need to dump on the corresponding port of the router.
Problems can be in the firewall, routing on your web server, NAT.

Answer 2

traceroute