I'm sorry you're having trouble browsing the internet using your Sonic 4g Mobile Hotspot. We definitely want to make sure you have the best experience possible with T-Mobile.
From your description, it sounds like the computer is still connected to the Mobile Hotspot over WiFi, you just cannot browse the internet anymore. We have some great troubleshooting steps in the Cannot web browse - Mobile Hotspot troubleshooting document that should help you stay connected.
Hope this helps!