Hello,
I just want to thank the OpenWRT / LEDE developers for their excellent work. I was able to install OpenWrt 18.06.0-rc1 without any problem:
- Activate ssh and change root password
- (A)DSL (AT&T/DSLextreme): Set VCI/VIP to 35/0
- WAN interface (PPPoE, username and password)
Interaction with the modem is way more responsive than the stock firmware, and who doesn't like packages and additional features. 
-Arne