# name of interface that should be monitored 
# (edit to match your interface name)
:set mInterface my_pppoe_connection;

# routing mark name used for default route that should be updated
# (edit to match your routing mark name)
:set mRoutingMark my_other_routing_mark;

/ip address ; 
:set mAddress [find interface=$mInterface] ;
:set mGateway [get $mAddress network]  ;

/ip route ;
:set mGatewayFound 0
:foreach i in  [find routing-mark=$mRoutingMark] do={ \
  :set mDstAddress [get $i dst-address]; \
  :set qResult [:find $mDstAddress "0.0.0.0/0"]; \
  :if ($qResult = 0) do={ \
    set $i gateway=$mGateway ; \
    :put "gateway rule adjusted"; \
    :set mGatewayFound 1; \
   }; \
}; 
:if ($mGatewayFound = 0) do={ ; \
  add dst-address=0.0.0.0/0 gateway=$mGateway routing-mark=$mRoutingMark; \
  :put "gateway rule added"; \
}; 

 /ip firewall mangle> print
Flags: X - disabled, I - invalid, D - dynamic 
 0   chain=prerouting action=mark-routing new-routing-mark=ostalo 
     passthrough=no src-address=192.168.50.0/24 dst-address-list=!free_zona 

 1   chain=prerouting action=mark-routing new-routing-mark=free_zona 
     passthrough=no src-address=192.168.50.0/24 dst-address-list=free_zona 

:local currentGateway [ /ip address get [/ip address find interface=interface_name ] network]

:local existingGateway [/ip route get [/ip route find comment="Comment on route which gateway we want to set"] gateway]


:if ($currentGateway != $existingGateway) do={

  :log info ("Setting default gateway for our pppoe interface " )
  :local a [ /ip route find comment="Comment on route which gateway we want to set"]

:if ([ :typeof $a ] = nil ) do={
    :log info "adding route..."
    /ip route add dst-address=0.0.0.0/0 gateway=$currentGateway routing-mark=the_name_of_the_mark comment="Comment on route which gateway we want to setK"
   } else={
    :log info "changing route..."
    /ip route set $a gateway=$currentGateway
   }
} else={
  :log info "No route changes needed."
}