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:

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:
  2. Wireless amplifier:
  3. Indoor dome antenna:
  4. Cables. Need two cables runs. One from outdoor antenna to amplifier and one from amplifier to indoor antenna: (we recommend this high quality low loss cable)
  5. Additionally recommend to add a lightning protector:
  6. Example Cellular router:

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