Used equipment security

[olaf-01]

I purchased a modem and router from a friend.
This morning his son texted me with my Internet speed, "238 download 14 upload Just ran a test on your WiFI 👀👀👀".
WTF! Not cool! How do I make sure he cannot get onto my home network.

[plemans]

What modem and router is it? 

Did you factory reset it and do a new install? 

That'd be the first thing I'd do with anything new. 

Push and hold reset for 7-15 seconds while its powered on, then do a new install on it. 

[labatt]

Make sure when you do the reset and reconfigure to use a different SSID and passphrase. If you use the same SSID for some reason at least change the passphrase.