Router or Repeater?

[Richard]

Go for Powerline networking (see Ken's answer). One Powerline adapter plugs in to a power outlet near your router (I suspect your modem is actually a router with an integrated modem). The other adapter plugs in wherever it is needed, and can provide Ethernet or WiFi connections. There are different models (and makes) of Powerline system and you can add extra adapters as necessary (upto a total of around 7 adapters).

WiFi repeaters slow down the connection you can achieve. A repeater is a store and forward device. It receives a packet from the router, and then transmits the packet to the computer. The same happens in the reverse direction. If the computer does not receive any signal directly from the router, then there will be times when both the computer and the router transmit together. When this happens, the repeater will receive corrupt information as the packets collide. Both the computer and the router will each wait a random time and then repeat the transmissions. This will further slow down the WiFi connection.

I hope this helps.