ScheduledWANDisconnect

"t-dsl" is the name of the WAN-interface, PPPoE in this case

/ system scheduler 
add name="sched_disconnect" on-event=pppoe_dis start-date=jan/01/2006 start-time=03:00:00 interval=1d \ 
comment="move daily T-DSL-disconnect to late night" disabled=yes

/system script
add name=pppoe_dis source={/interface pppoe-client disable [find name="t-dsl"]
:delay 10
/interface pppoe-client enable [find name="t-dsl"]
:log info "Script pppoe_dis - scheduled T-DSL-disconnect executed."
}

should be tested in your environment and use it by own risk!