Have second hand dlink dsl502t

Question

have second hand dlink dsl502t modem am trying to access modem to change settings. the modem is unsupported by my isp but they have told me it will work but i need to access the modem and enter my user name and password. i have tried the address 10.1.1.1 but in order to get there i need a username and password. i have entered mine and the previous owners details but still no access. is there any other way to access the modem?.