>The router is forwarding all incoming traffic on port 8080 to this IP addres
So you want Apache to be accessed/reached on 192.168.15.100:8080 and *not* on 192.168.16.100:8080 (as written above). It is very hard to help, if informations differs from one post to another.
Ok, I will assume the following:
a) 192.168.15.0/24 is connected via Router to the internet
b) Apache is running on 8080, all internal IPs. You might specify more precisely the Listen-Directive to "Listen 192.168.15.100:8080"
c) you have a portforwarding on your router, which forwards Port 8080 to 192.168.15.100
So far ok?
So you *should* have access to Apache via http://192.168.15.100:8080
from your LAN. Does that work?