How to roll back the MN700 firmware? New firmware have bug for PPPoE dial.

When I use MN700 as router, it often went to idel after
recieve packets from hundred's IP address, such peer to
peer application. When I update it to the latest
firmware, it can't login via my ADSL (PPPoE) in router
mode. I only can use it as an access point and login to
ISP via my PC.
Who know how to roll back the old firmware? In old
version, I still use it as router even it often go to
idel.