There are several apps that you can try to help your phone have better signal strength. I haven't used any of these apps so I can't tell if they really work. But I they're worth the try. Let me know if any of these apps work.
1. Network Signal Speed Booster
2. WiFi + 3G Signal Booster
3. Signal Booster for Android
4. Signal Booster 2X - Ultra Fast
5. Signal Boosters
6. 4G 3G Signal Booster Pro FREE
These apps are free to download from the Play Store.