As far as I can tell, the guest network is just for devices connected by Wi-Fi. Is it possible to add devices that are connected by wired ethernet to the guest network?
Yes, need to set up vlan. This depends on which router you have.
You can find in luci actually.
Go into luci and you can change which network the wired LAN is connected to
In my case I have 1 AR750S Slate and 2 AR750 Creta. I have 2Ghz and 5Ghz wifi for all of them with same name, also Guest wifi with same name so you change from router to router with no delay. All of them wired. LAN zone for Private wifi, Guest Zone for Guest wifi in Firewall.
Guest : You have to make a new vlan in luci>network>switch for example 10 in all the routers, then select tagged in CPU and LAN in the LAN port the wire will be connected between them,
Bridge this vlan to Guest in luci>network select Guest interface (eth0.10 in this case) and Edit and then bridge also to the Guest wifis also in all the routers. I have STP enabled for all routers also,
Main router has interface Guest static IP for example 192.168.9.1 and enable DHCP also, Firewall to Guest Zone.
Other routers will be for example 192.168.9.2 and 192.168.9.3 with Gateway and DNS Server IP 192.168.9.1. NO DHCP ENABLED. Main router will give IP.
I think that’s all