I was only looking for that tab to activate the 'Block Internet Access for specific IoT devices.
Generally, in the Merlin or stock Asus firmware (for the RT-AC68U), one clicks on the client entry in the Network Map > Client Status section and they'll get a popup where one can enable the Block Internet Access option on that specific client.
If one isn't seeing a client listed in the Client Status one can try manually blocking access by accessing the Parental Controls > Time Scheduling section.
General steps: Enable the Enable Time Scheduling option. Then enter or select the MAC Address for the client you want to block access for. Then set the drop down box from Time to Block and click the Add (plus icon) on the right side to add the client to the list. Once added to the list, click the Edit icon under Time Management for the client entry and manually configure the time/day you want the block to occur. Then click the Apply button at the bottom to save the settings.
Edit to add: If using Merlin firmware with the YazFi add-on, then the wireless YazFi guest client not showing up in the Network Map is standard behavior. When using YazFi, the wireless guest clients will be listed under System Log > Wireless Log. Or they will show on the YazFi tab under the Guest Network section or under the YazFi CLI interface.