Server with Static IP gets DHCP'd by router

[mahesh]

Hi all - I am a "n00b" and just started with Ubuntu (and Linux in general). I wanted to build a little server for my home office. I had an old Mac mini, so I installed Ubuntu Server 22.04.1 LTS on it. Worked like a charm. I then installed XFCE because I like to have some color on my screen

The server has a static IP. I entered it during installation. It is 192.168.2.5. I definitely did not turn on DHCP.

My router (a Speedport 3 from German Telekom) however decided to give the machine 192.168.2.165.

I checked netplan, there was no mention of DHCP in the config.yaml. It seems to be absolutely not turned on. ifconfig on the machine shows me *.5. Pinging the machine also works only when using *.5 - there is nothing at *.165. But the router insists.

In order to make quite sure, I did "sudo apt-get remove isc-dhcp-client" and "sudo apt autoremove" to make sure DHCP was not only inactive, but physically not on the server. I rebooted the machine - and saw the *.5 show up in the router list. For a minute. It then vanished and was replaced by the *.165 once more.

The server as such works fine, I can connect to the server from within my network. The server is also able to connect to the outside world. However, I cannot reach the server from outside, because in order to do that, I would need to be able to tell my router to open a port to that server. Which is not possible, because the router insists the server is *.165 instead of *.5.

Can anyone help me with this? I now spent several hours trying to find out what might be wrong here.... there is nothing on the server except XFCE and apache2 (yet). What might be causing this?