WI FI Sharing - using as modem for computer - explain how it works

New to the world of tablet with Dell Streak 7 purchased May 21. Great devise for the price and features offered. As a standalone device, I love it. As a wi fi sharing using as modem for computer compared to broadband service which I cancelled, I need to understand better the pros and cons of using wi fi.Connection for internet access has been erratic so I'm trying to understand if there's a special setting that needs to be calibrated, or whether it's more where one lives and the reception that is being received. What recommendations do you have to ensure reliable wi fi sharing connection. Made 4 calls to support, went to store where I bought for additional tips. It works at store...but in my location, major city, it's not connecting to intenet consistantly. It says excellent reception. Dell Mobility support hasn't been helpful to identify problem. Store put in new sim card. Need recommendations for extending battery life or is there separate accessory for charging back up battery