Ok, first you have to open the router's page by writing the ip address of the router in the address bar of your browser, I assume it's 192.168.1.1, you can find it by looking at what gateway any device gets from dhcp. Then click on WInbox and a program will download, after that fill the connect to with the same ip that you put in your browser, username is by default admin and no password, then click connect.
In the new window click on New terminal (on the left) and another window will apear inside the current window, select all the code bellow(all of it not just bits) copy it (CTRL+C) and paste it the terminal window(right click and paste, DO NOT USE CTRL+V IN TERMINAL!! ).
If you have not modified the names of the interfaces it should work. It is made aspecially for you! on IP 192.168.1.36.
/ip firewall filter
add chain=input in-interface=ether1-gateway protocol=tcp port=80 place-before=0
add chain=input in-interface=ether1-gateway protocol=udp port=88 place-before=0
add chain=input in-interface=ether1-gateway protocol=tcp port=53 place-before=0
add chain=input in-interface=ether1-gateway protocol=udp port=53 place-before=0
add chain=input in-interface=ether1-gateway protocol=tcp port=1863 place-before=0
add chain=input in-interface=ether1-gateway protocol=udp port=1863 place-before=0
add chain=input in-interface=ether1-gateway protocol=tcp port=3074 place-before=0
add chain=input in-interface=ether1-gateway protocol=udp port=3074 place-before=0
add chain=input in-interface=ether1-gateway protocol=tcp port=3075 place-before=0
add chain=input in-interface=ether1-gateway protocol=udp port=3075 place-before=0
add chain=forward in-interface=ether1-gateway protocol=tcp port=80 place-before=0
add chain=forward in-interface=ether1-gateway protocol=udp port=88 place-before=0
add chain=forward in-interface=ether1-gateway protocol=tcp port=53 place-before=0
add chain=forward in-interface=ether1-gateway protocol=udp port=53 place-before=0
add chain=forward in-interface=ether1-gateway protocol=tcp port=1863 place-before=0
add chain=forward in-interface=ether1-gateway protocol=udp port=1863 place-before=0
add chain=forward in-interface=ether1-gateway protocol=tcp port=3074 place-before=0
add chain=forward in-interface=ether1-gateway protocol=udp port=3074 place-before=0
add chain=forward in-interface=ether1-gateway protocol=tcp port=3075 place-before=0
add chain=forward in-interface=ether1-gateway protocol=udp port=3075 place-before=0
/ip firewall nat
add chain=dstnat protocol=tcp dst-port=80 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=80
add chain=dstnat protocol=udp dst-port=88 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=88
add chain=dstnat protocol=tcp dst-port=53 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=53
add chain=dstnat protocol=udp dst-port=53 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=53
add chain=dstnat protocol=tcp dst-port=1863 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=1863
add chain=dstnat protocol=udp dst-port=1863 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=1863
add chain=dstnat protocol=tcp dst-port=3074 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=3074
add chain=dstnat protocol=udp dst-port=3074 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=3074
add chain=dstnat protocol=tcp dst-port=3075 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=3075
add chain=dstnat protocol=udp dst-port=3075 in-interface=ether1-gateway action=dst-nat to-addresses=192.168.1.36 to-ports=3075
#END OF SCRIPT IN CASE YOU FORGET TO PRESS ENTER AT LAST LINE
I have tested this on a 750U and it works only I used different subnet. Make shore your xbox network settings are correct.
You can see these rules and modify them in winbox at IP-> firewall -> filters rules and nat