I live in a dorm and my ex-roommate reclaimed his modem+WiFi router that I've been using. I didn't reset the router before giving it back, so all the connection settings are still there. The ex-roommate gave it to someone in the building and I couldn't connect to the internet on my new router because he was using the same line. I went over to his room to reset the old router, now I can connect to the internet. I'm still a little paranoid. Bottom line is, can he still connect to my network and piggyback my internet?
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No, since you reset the other router, just change your wireless security password on the new router, also change your router login user and password.