Ex-o2 FRITZ Box 6660 Cable: enabling LAN 1 as WAN via a bootloader variable

My internet comes from the wall socket of a building network that I share with a lot of other people. My router is a FRITZ!Box 6660 Cable, an ex-o2 box (article number 20002933) that I own. It was in IP-client mode, which means it only passes that network through: my laptop, media devices, home server and printer each got their own address on the shared network, and anyone else on it could reach them. I wanted a private network that sit behind a NAT and router firewall.

I started in Claude Code with Opus 5.5 on Medium effort. It scanned my subnet and recommended router mode with LAN 1 as WAN. But the LAN 1 WAN option was missing from the web interface. I told it the box was an o2 variant I own and asked it to install the latest firmware.

It declined, saying you can't install AVM's standard firmware on a provider cable box the normal way and that the hacks to change the branding can brick it. It kept suggesting buying a cheap router and putting it in front of the FRITZ!Box (what?!). When I said I didn't want to buy anything else, it gave me unnecessary steps (firewall rules per machine, an email to the network admins) and refused to unlock the box. It never checked what firmware the box was running for some reason. I mean I could have started a new session but I was bored at that point.

Two days later I tried again with pi, running GLM-5.3 through Ollama's cloud and dude did not ask a single question.

Over TR-064 the box reported FRITZ!OS 7.57 with the model string "6660 - avm", so it already ran AVM-branded firmware and needed no flashing. From German forums and the DieFlashe docs GLM found why the menus were missing. A bootloader variable called DMC was set to RTL=n,SL1, which marks the box as a non-retail provider device. That flag hides System > Update, and on my box also Internet > Account Information, where the LAN 1 WAN option lives. The documented fix for the 6660 is to set DMC to RTL=y,SL1 through EVA, the FTP service the bootloader opens for a few seconds during boot. No firmware gets written.

macOS has no ftp client, so GLM wrote a Python script that polls the bootloader, catches the EVA window and reads or sets the variable.

EVA needs a wired connection, so I bought an ISY USB-C Ethernet adapter. I plugged into LAN 1 and power-cycled the box and EVA never answered. GLM then found a solved IP-Phone-Forum thread from July 2026: on the 6660 Cable the bootloader doesn't answer on LAN 1, only on the other ports. On LAN 3 the script got the FTP banner on the first boot.

The script read DMC=RTL=n,SL1, wrote RTL=y,SL1, read it back and rebooted. Update and Account Information were both back. I saved a settings backup and uploaded the official FRITZ!OS 8.25 image under System > Update (FRITZ!OS File tab), and the settings were kept. Then I set LAN 1 as WAN in router mode. Wohoo! My own private network, at last.

Thanks to the people at IP-Phone-Forum and DieFlashe for documenting this.