We don't know what modem/router and wifi you are using. If you have a modem/router connected to your internet line, but you have a separate wifi router plugged into the modem/router, then this situation can arise. (If it's all one device, wifi and LAN in the same box, the following is not relevant).
The modem/router hands out IP numbers to the devices connected to it, including the wifi router. If the wifi router is also set to use DHCP, it will also hand out IP numbers. Apart from the fact that this may cause a clash (with the same number being allocated to two different devices) anything connected to the wifi can't see the devices connected to the modem/router.
The wifi router needs to be set in 'bridge mode', so that IP numbers handed out by the modem/router are passed through to the wifi connections.