Last week I wired up my home network (including custom modem and routers) myself, because the stuff my ISP wanted me to use was garbage.
Luckily Germany has "router-freedom" so ISPs are not allowed to force us to use their device to dial into the network.
I did everything myself, because the 'technicians' they kept sending me were just idiots who didn't know anything, considering the highly paid job they are doing. Usually they told me, to get the device from my ISP, because my "Router" (actually a business grade, standalone Modem by Cisco, to feed my Router) didn't even have WiFi ( lol ). Also all Technicians didn't arrive at the agreed date but at some other time. I wasn't able to wait any longer.
So I did it myself.

Consider me something more like a student of theoretical computer science. Not actually supposed to be experienced with hardware stuff.

The ISP is serving me with a DOCSIS 3.0 Network based on the television cable network in my city. For some reason they are providing the internet-access to only one socket in the apartment, which has a rather uncommon "WICLIC" connector. After having trouble getting an adapter for WICLIC to common coaxial F-Connectors (used by every DOCSIS-Modem), I made one myself.

After setting up everything (not that hard, once the connectors fit) my modem told me, that, while I'm perfectly connected to the ISPs internal Network, I still can't access the internet.

So I called the ISP...
After getting ranted at, about that what I'm doing is illegal and only certified employees are allowed to do this and I will break more, than actually do good and that I can't just connect my own "Router" (again I needed to correct her: Modem) I hang up the phone.
Also she accused me of hacking their devices because I'm not supposed to see my IP address... (My Modem told me on its web interface. I didn't even need telnet for that.)
I went to the ISPs head office, told the first desk as many technical terms as I could remember and got forwarded to something like the main technician.

He was a really nice guy. The only sane and qualified person I dealt with at this company. He asked me for my Address and Device Model, I told him my MAC and last internal IP, I had seen and he activated my internet access within a minute.
We talked a while about the stupid connector that ISP is using in the homes and he gifted me some nicer adapters to connect my modem to the wall.

Why do ISPs hate their customers that much?