Many modern routers support two frequency bands, a 2.4 band and a 5 GHz band. It is like having two routers in one. It is easy to set one of those up as a hidden connection, while the other has an SSID that shows up in your list of possible network connections. If you unplug the router, both signals will go down.
“a 2.4 band” > “a 2.4 GHz band”