If you are reluctant to give out your WiFi password to your guests who want to borrow your WiFi to get their mobiles devices to connect to the Internet when they are visiting your place, you can set up a Guest Network for them.
Guest Network is a useful function to allow guest or public Internet access while separating them from your own internal network. Asus Guest Network also has some advanced features for you to impose restrictions such as access time, bandwidth limiting, MAC filter, etc.
The Guest Network function in Asuswrt-Merlin firmware should be the same as in original Asuswrt firmware. It allows you to configure up to 3 separate Guest Networks for 2.4GHz wireless band and up to another 3 separate Guest Networks for 5GHz wireless band.
If your Asus router has two 5GHz wireless bands, then you'll have yet another 3 separate Guest Networks for your second 5GHz wireless band.
It is advisable for you to set an SSID for Guest Network that is different from your regular WiFi SSID.
If you set the Authentication Method to "Open System", anyone around your wireless router can connect to your Guest Network without the need of any password. It is advisable to set the Authentication Method to "WPA2-Personal" and set the WPA Pre-Shared Key as the password to connect to your Guest Network, which can be different from your regular WiFi password.
You can optionally restrict the Access Time of this particular Guest Network. For example, if you set the Access Time to 2 hours, the Internet access for this Guest Network will be cut off 2 hours after you clicked on the Apply button.
You can also optionally limit the Download Bandwidth and/or Upload Bandwidth for this Guest Network. If you set the value to 0 Mb/s or higher than your Internet access bandwidth assigned by your ISP, then it is not restricted.
If you enable Access Intranet, the devices connected to this Guest Network is able to access to networked devices connected to any of the LAN ports of your Asus router. If you use a switch to expand the network linked to a LAN port, all the devices connected to the switch are also accessible by the Guest Network. In this way, your Intranet is exposed to your guest.
If you disable Access Intranet, your guest won't be able to access to any networked devices connected to the LAN ports of your Asus router. However, they can still access to the devices connected to your WiFi, including those connected to your regular WiFi network.
If you want to also disable access to your WiFi connected devices, you can Set AP Isolated for a particular wireless band under Advanced Settings > Wireless > Professional. However, this setting is not Guest Network specific, and will affect all the devices connected to that particular wireless band.
If you want to only isolate the Guest Network from other wireless devices, you can explore into YazFi expansion for Asuswrt-Merlin. You can click here to find out more information about YazFi expansion in SNBForums, including its installation method and sample configuration file.
You can also optionally make use of MAC Filter to specify which mobile devices is allowed or not allowed to connect to this particular Guest Network.