I recently got a MiFi card for my on-call rotation with work and had some issues getting it to play nice with the Ubuntu install on my laptop. It’s a Samsung SCH-LC11 and various iMac’s in the office connected to it just fine. My laptop would connect, then almost immediately disconnect. Pretty much unusable. I connected to the office wifi just fine so I know that wasn’t an issue.
A quick search found a solution on the ubuntu forums. Basically, you need to connect to the device (obviously using a Windows or Mac) and log into the web admin page (default of 192.168.1.1). Check the wifi configuration security; the encryption protocol is probably set to WPA with TKIP. You need to set it to WPA2 with AES.