You never did say what brand modem it was, but given your location, and the mention of an 'activity' light, it sounds like you have the Speedstream 5100 modem that SBC/Ameritech used prior to becoming AT&T.
The unlit activity light only means the comp and modem are not currently transmitting or receiving data. Since the other lights are green, it still can be several things:
- your computer or NIC card
- bad or improperly plugged in DSL line filters in the house.
- defective modem or power supply
- line quality issues, or
- corrupted user account on ATT's access server.
Before the tech arrives, visit all of the wall jacks in your house. Ensure no one plugged a phone directly into the jack with no filter. You can also unplug all the phones in the house, and see if the surf problem clears. This will isolate a defective filter. Ensure the modem is plugged into the proper side (DSL/HPNA) side of its filter.
When the tech comes by, in addition to checking the line, ensure he plugs in his workstation (essentially a Panasonic Toughbook laptop) into your modem and attempt to surf. This will verify or eliminate your comp as the source of the problems.
Unless you are paying monthly for your modem, they may try to bill you for a replacement if yours is out of warranty. Just a word of caution.