why sometime i have to enable Legacy mode, inorder to get access to my router and sometimes not ..?
so when i get this message "the router doesn't support secure connection, Please enable leagacy mode. what that mean..?

Sometime around ROS v6.42 (I don't remember exact version) passwords in ROS started to be stored crypted, before that they were plaintext. Consequently the login procedure had bo be changed. Now the "legacy" refers to old login procedure ...

In other words, sounds like you have one or more routers with really old firmware. Should be updated. There are some pretty serious security holes in the older versions - if for no other reason.

I see this on my Chateau LTE12 sometimes when I factory reset it.