I can't be specific about version 15 as I haven't "gone there" yet. On 14 you are given 3 different options about how your network is set up. The option I used was one with no FQDN, which meant that the local IP is used for local devices. However, a bit farther down the road, you are asked for, no, insisted on, a Public IP. I put my current public IP in there, but later changed it to my DDNS URL setting. Again, not supported by 3CX. You will have to see which of the install options works in your case. Pretty much any datafill you enter, can later be changed under (in version 14) Troubleshooting/ parameters.