If you wish to provide Internet access to others using your router but don't want them to have access to your network then you can create a Guest Wifi to achieve this.
Go to the Network menu and then to the Guest Wifi submenu.
The first step is to select which wifi radio you wish to use. If your router doesn't have both 2.4Ghz and 5Ghz wifi then there is only one choice here. Check the Wifi submenu of the Network menu to see which radio is assigned to which Wifi frequency. Normally, in dual frequency Wifi, radio0 is assigned to the 5Ghz Wifi but that can vary in some routers..
Next you set up the actual Wifi interface.
Chose a Network Name for this Wifi that the users will see on their devices. This can be any name but spaces will be replaced by the underscore.
The Network IP Address must be in a different subnet than the router in order for this interface to work. The router by default uses the 192.168.1.x subnet. By default the Guest Wifi uses the 192.168.3.x subnet but this can be changed if need be.
If you are not interested in limiting the Internet speed on this interface you can skip the next section and just click on Save&Apply to finish.
If your router uses a 4meg image then it does not support Bandwidth Speed Limiting for the Guest Wifi.
By default the Bandwidth Speed Limiting for the Guest Wifi is disabled.
Select Enable and the speed limiting parameters will appear.
Enter the Download and Upload speeds you wish to limit the Guest Wifi to and then click on Save&Apply.
The Wifi network that you have just created is not encrypted so you may want to do so at this time. Go to the Network menu and the Wireless submenu and click on the Edit button for the Guest Wifi interface. Scroll down to the Interface Configuration section and click on the Wireless Security tab.
From here you can set up the encryption for the Guest Wifi network.
Your guests will now find a Wifi network that will allow them Internet access but will not allow them to access your Network.