The WiFi network could be hidden. If the name is hidden in the router/modem settings it won't show on the device. It also could be the router putting out a different GHz. There are 2.4 and 5. So depending on which is on that router/modem and what your device needs, that could be it.
If it isn't that. Try turning off your WiFi, restart the phone and then turn on the WiFi and see if it shows. If it doesn't, and other WiFi in the area does show, then it isn't the phone.