I have a l2tp tunnel over a unstable wan line.
The connection breaks often, but MT fails to restablish it. I found out the problem is that IP is not removed properly on the client.
As a workaround I have this script scheduled every minute.
It checks if the IP of the tunnel is being used when the tunnel is not running, then removes it.
:if ( [/interface get l2tp-out1 running] = false ) do={
/ip address remove [find address="192.168.100.2/32"]
:log info FIXING_STALE_IP
}