Hello Andy,
There are no servers for gameplay on Monster Hunter Tri. We do use servers for matchmaking but that is about it. The online experience is based on peer to peer service. This means the quality of the experience is based on the connection quality between you and the people you play with. If you are experiencing connection issues try the following steps below.
Connect directly to your broadband modem
If you have to use a router connect your Wii to it with a Wii ethernet adapter.
If wireless is the only way to connect to your router then disable your firewall.
If this issue continues or you are unable to disable your firewall then set your router so it has the Wii in a DMZ.
Also see Nintendo's support site for more tips on preventing disconnects.
CAPCOM Customer Support