RAIN MiFi Model R811A very unreliable

I bought the MiFi Model R811A for Rain, the device loses network connection every 30 minutes. I have to restart which is frustrating. I have logged numerous calls, I had it exchanged 3 times and the device behaves the same. The sim card works well on other devices. Please pull out this device if you know it doesn't work and no one has explained to me what is wrong with the other collected devices. I was told to have it replaced with another model I have to order another one.