Mario Kart Unlimited - Unlimited Mario Kart Wii Experiences!

Wii / Wii U / Switch:

[UDP Ports]
~All Games~
1-65535
OR
1024-65535

~Mario Kart Wii Only~
22XXX (where XXX is the last 3 digits of your console's serial number)

[TCP Ports]
Wii U / Switch:
6667
12400

Wii / Wii U / Switch:
28910
29900-29901
29920


You can also check for the fastest DNS server using namebench.
And you can optimize your router's MTU by reading this guide.