Using your own router behind "Glasfaser Modem 2" is really simple. It's just that Deutsche Telekom aren't very helpful.

There's a nice guide here for OpenWRT routers. The basic steps apply to any router:

• Use PPPoE for the WAN interface
• Use your Internet-Zugangsnummer as the username
• Use your Persönliches Kennwort as the password
• VLAN-tag your internet traffic with 7 (and IPTV with 8)

Every router has these settings in different places, most of which make little sense to me. Below, I'll show what you need to do on three different routers: