Are you saying your smart device is displaying two different sets of SSID's - one for the old gateway and one for the new gateway:?
I recently updated and replaced my modem and the name of my WiFi. However, the old WiFi that isn't working anymore still appears on the list of my available WiFis. It will sometimes default to one of my old ones and of course, can't connect. It appears on my iPhone, my iPad and my iMac. How can I delete the old, useless ones? I also have a separate router.
Can you find the "source" of the SSID's? You say you have a router; could it be the source?
I can assist you. Please send me a private message with your full name for verification, and account number by clicking on my name and then "private message me" and we can get started.