Pppoe-wan failure in every reboot

hello friends
i have an issue on my openwrt router that is pppoe-wan can not connect to ISP in every reboot
it will connect if i restart network service or if i run ip link set wan down; ip link set wan up.
i called isp and they said after reboot they can see the disconnection but nothing else , it means there is no try from the router for pppoe connection after reboot.
i solved my problem by a crone script to check wan status every 15 min and do the ip link set down and up , but what is the root cause of this failure ?

Examine logread / loci/status/log

go through luci and add to Autorun
( sleep 10 && /etc/init.d/network restart )
( sleep 5 && /etc/init.d/firewall restart )
exit 0

the reason itself may be related to either the provider or the settings