Related to the old well known issue that the connection from WinBox to the router is sporadically disconnected with the message “router has been disconnected”:
I have found the relation of circumstances.
Winbox is disconnected each time that a device connects to WiFi and an IP address is leased over DHCP. Maybe this is related to DHCP only.
This problem have been there for years after I upgraded RB2011UiAS-2HnD-IN from 6.34 to any of the next versions. 6.34 had this issue not at all.
I have a comparison: at one place there is still a RB2011UiAS-2HnD-IN with 6.34 and it has no problems (and I won’t take risk of upgrading it because I don’t have a physical access to it).