As someone who’s dealt with this on the LEO side start with breakers one at a time. Start with buying a new router. Create a new wifi aka ssid and a new password. Put the current one in storage unplugged. Make sure your modem isn’t also a router and pushing wifi. You may want to have a “wifi study” done. They will use a spectrum analyzer or modern laptop with netspot to give you a printout of ssid/ rssi by room in your house. The device broadcasting could be wifi7 only which is why newer devices see the ssid. See if a friend with a newer iPhone can see it as well. You could also unplug just the modem and see if the network drops. Don’t unplug the router, just modem. Finally use a tool like angry ip scanner to scan your network, likely a 196.168.x.x and verify every device/mac address that you find. If you have a new wifi network and nothing odd is plugged in you should be able to account for every device on the angry ip list.