Nee. Dat kan niet.

Dear ​@sncr 
What you want is unfortunately not possible.
You can place the KPN router and connect the Edge router behind it as an access point.
You can also connect the Edge router to the fiber optic point via a LAN cable and store the KPN router neatly. If you then have a connection problem, KPN would like you to connect the KPN router to investigate the problem.
There are a number of links in this forum to connect your own router, but unfortunately only in Dutch.
Good luck.

I established a PPP connection on VLAN 6, and the EdgeRouter successfully obtained an IP address for internet access — that part works fine.

Then, I connected a cable running to the living room and wanted to use the KPN Box 12 there as a Wi-Fi access point, since the Wi-Fi signal from the router inside the meter cabinet is weak.

However, if the KPN Box 12 doesn’t support bridge mode or a Wi-Fi access point mode, it’s not useful for my setup, and I would prefer to return it.

Hello ​@sncr 

You could try if the Box 12 works as an AP by connecting the EdgeRouter with a Box 12 LAN port after having disabled the DHCP server in the Box 12 router.

Depends on the firmware where you can disable DHCP:

You should not return the KPN Box12 while having a subscription, in case of internet connections problems KPN customer service will almost always ask you to connect it so they can analyze any problems on the connection.

The KPN Box 12 doesn’t allow changing anything—forget disabling DHCP, I can’t even change its IP address. It’s ridiculous. I’m sure if this home gateway device had firmware that wasn’t customized for KPN, it would allow these things. At the very least, I could set it to bridge mode or change the WAN port’s connection type from PPP. The whole configuration seems hardcoded, while pretending to offer a configurable interface to the user.

Anyway, here’s the workaround.

Since the KPN Box 12 only supports PPP on VLAN 6, I set up a PPP server on my own router over VLAN 6. That solved the problem.

I wish it didn’t have to be like this, but for now, this setup is usable for me. Until I get a new router, I’ll manage with a PPP connection between my router and the KPN Box.