iPhone Reset

You need either a cellular data connection or a Wi-Fi connection to disable Activation Lock from the phone.

Start by following this Apple support article—>What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If the first part doesn’t work use the section on what to do if you no longer have the phone.

maybe check your internet connection and make sure you have a good one. Or try resetting your phone using a PC.

Good luck!