Few solutions but always Build In are the best.
/tool traffic-monitor add name=CheckPPPoE interface=pppoe-out1 traffic=received trigger=always threshold=0 on-event=SystemScriptNameToRun comment="ThanxToSiB" disabled=no
and in System Scripts this name=SystemScriptNameToRun:
/interface pppoe-client enable pppoe-out1;
This is ReEnable means create new connection.
All this can be more complicated but I hope this method will be good.