Which plugin do I need for the GL-MT300N-V2 to detect my Realtek RTL8192CU Wireless LAN 802.11n USB 2.0 Network Adapter wifi dongle?

Routers
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If the USB wifi adapter is detected and working, then it should show up on the Admin Panel → Wireless page. You can configure the GL-MT300N-V2 as a Repeater by following the documentation: