I got my son’s HomePod (used) and it’s not connecting to the wifi

• It should connect to the same wifi the iOS device you used for set up. HomePod uses the Wi-Fi network that your iOS or iPadOS device is connected to. 

Set up and use HomePod

Check that two-factor authentication is turned on for your Apple ID and iCloud Keychain is turned on

• To check if two-factor authentication is turned on, go to Settings > [Your Name] > Password & Security. If two-factor authentication isn't set up, select the option to enable it and follow the steps.
• To check if iCloud Keychain is turned on, go to Settings > [Your Name] > iCloud. Scroll down and check that Keychain is on. If it isn't, tap Keychain and turn it on. If prompted, follow the steps to set up iCloud Keychain.

Please also see this: if you can't set up HomePod.