I agree with Drew: the problem is probably on the client side, not OnStep's WiFi server.

My phone is from 2014, and it works well.
In the past I have seen devices that refuse to connect to a certain router.
The router was old, and it was time to replace it, so I did, and the problem went away.

Try another phone make/model, see if it works.

Whatever you do, do not change anything in the code. Even the addons/WiFi/Config.h
file almost never needs to be changed.

Also, if you want to experiment, there are Examples in the Arduino IDE for the ESP8266
board manager. One of them is WiFiAccessPoint. Try to load that example sketch to the
WeMos, and see if it works or not. They are under File -> Examples -> ESP8266WiFi.