To improve your USB Modem cell signal you may consider attaching an external cellular antenna or using a cellular amplifier/signal booster system. The low cost and most efficient is using an external cellular antenna that is directly attached to your USB modem.
External cellular antennas include; directional antenna kits such as RFWY12-USB800-020 with 20 ft low loss coaxial cable, RFWY12-USB800-050 with 50 ft low loss coaxial, and RFWLPCAB240PIGTS9 (with 20 ft coaxial cable).
For a custom signal improvement solution, Please contact an RFWEL Wireless Indoor Coverage Specialist.