The Sprint/ Franklin Wireless U300 doesn't have an external antenna port so the only two methods to attach an external booster is either to use a passive antenna coupler (NOT recommended) or use a cellular repeater (the recommended method).

Here is an example drawing of the recommended setup: http://www.rfwel.com/support/tech-dr...wwan-split.pdf.

In actuality if you just want it for the U300 in a single room them you can simplify the setup. Also we show it here using a Cellular router which is optional but convenient if you want to share the internet connection with several computers concurrently. Here is what you would need as a minimum:
  1. Outdoor high gain directional antenna: http://www.rfwel.com/shop/Yagi-1800-...S-Antenna.html
  2. Wireless amplifier: http://www.rfwel.com/shop/Wilson-Ele...r-1900MHz.html
  3. Indoor dome antenna: http://www.rfwel.com/shop/Dual-Band-Dome-Antenna.html
  4. Cables. Need two cables runs. One from outdoor antenna to amplifier and one from amplifier to indoor antenna: http://www.rfwel.com/shop/400-Series/ (we recommend this high quality low loss cable)
  5. Additionally recommend to add a lightning protector: http://www.rfwel.com/shop/Lightning-...0W-230VDC.html
  6. Example Cellular router: http://www.rfwel.com/shop/CradlePoin...nd-Router.html

This is a very high performance solution and will completely fix your problem.