How to set up wired access point for QoS?

[pbix]

You must place Gargoyle in router mode to use QoS

[pbix]

You cannot use QoS unless you are using the WAN port. You cannot use DHCP from your primary router when using the WAN port to connect to it.

Having Gargoyle in gateway mode means you want Gargoyle to create its own LAN.

Fundamentally you cannot do what you say you want to do. Enable DHCP on your Gargoyle router, connect the WAN port to your primary router. Accept that these must be different LANs. Then you can use QoS.

Or put Gargoyle on the primary router.

[pbix]

If your printer has a fixed address on the primary LAN then all computers will be able to access it.

On an note related to the QoS questions you seemed intent an previously any device connected to the primary LAN will not be subject to Gargoyle's QoS.

You linked to a post which does not discuss or use QoS. If you do not want to use QoS then you can make a repeater easily using the pick on the Gargoyle UI. Such a link is confusing to me. Do you want to use QoS or not?