Unable to Log onto my wireless DSL via laptop

Hello, I have a Dell desktop ~ About a year ago I upgraded my AT&T high speed connection to wireless so visiting family members could use their laptops ~ We have tried just about everything & no one has successfully been able to use wireless/with their laptops ~ They have the key ~ they have the wireless network password etc etc ~ I have called AT&T tech support with no luck ~ A friend suggested my modem didn't have enough power ~ I am quite stymied ~ Can anyone offer me any insight ~ ? ~ Much appreciated ~ !!!