connect the modem via Ethernet cable, open internet explorer and type 192.168.1.1 in the address bar, that gets you in the modem's settings, go to wireless security section and see what the password is, or change it to anything you want.
if the modem asks you for a user name and password it's usually username: admin and the password one of these: password or admin or leave it blank.
try this and see what you can do
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