1. Make sure there's nothing in between the modem and where you want to receive signal, e.g. a metal or thick concrete wall. Ideally the modem should be beside a window in line of sight of where you want the signal to reach.
2. Consider looking for antenna upgrades for your modem such as a directional antenna to boost the signal.
3. Possibly look for better router that already has much better antennas built in.
4. Most expensive would be to attempt to daisey chain wireless units together to increase the radius of your wireless network. E.g. setup an access point a few hundred feet away from your current modem, providing that the signal reaches clear enough.