If I purchase a phone from someone on an AT&T plan, and they have not yet paid off the device, can I just pop out the AT&T sim and pop in my ST sim and it work?

I had my internet service shut off, but paid my bill and am all caught up. My account even has credit on it. But my modem is still not connecting to the internet even though the green lights are all showing appropriately, I've done a hard reset, unplugged and replugged, restarted the other device, n

Rolling Charge are Cramming by At&t. Here are the educations, I expereinced currently. My July end billing date is the 5th. My new august bill starts July 6th. every month 5/6th are billing migrations dates. So, on the 6th of july, [AUGUST] bill showed [unpaid[ amount of $39.85. also, this was the [