config interface 'wan'
option proto 'pppoe'
option username 'user@t-online.de'
option password 'pass'
option ifname 'eth0.7'
option force_link '1'
option ipv6 'auto'
option delegate '0'
config interface 'wan6'
option ifname '@wan'
option proto 'dhcpv6'
option reqaddress 'try'
option reqprefix 'auto'
Edit: Once working, give it a day to settle. Your ISP's DSLAM notices the frequent disconnects and doesn't like it, or rather adapts by reducing link speed to counter the 'unstable' connection.