This article describes what to do if the red wireless service LED is solid red.
INSTRUCTIONS & INFO
A flashing red signal strength indicator means that the SIM card is missing or not installed properly.
Remove the bottom cover of your AT&T Wireless Internet and the back-up battery. Make sure the SIM card is pushed all the way into the SIM card slot and is secure.
SIM card may be invalid or damaged and need to be replaced. For a replacement SIM card, take your AT&T Wireless Internet device to your local AT&T store. Or, call AT&T Customer Care at 1-800-331-0500.