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.

This can also be because of configuration issue (in my case overlapping networks and OSPF) - I got this error when the route was available only to the destination router and not back to the place from where you try to connect.

Try traceroute to the destination network and then from a device on the other side back to the source (if you have a chance).

I have seen that as well after "reset configuration".
I think it is a timing problem. The router is booting, at some time it accepts a TCP connect but is not ready for the actual login, and then winbox mistakenly decides that it must be a legacy device because it does not complete the new-style login sequence.