The Alfa AWUS1900 uses the Realtek RTL8814AU chipset, so it needs an OpenWRT driver package with name similar to kmod-rtl8812au. I could not find such a driver on my Brume, nor Slate routers.
You may have better luck with a wifi adapter using the MediaTek MT7612U chipset, which works on my both my Slate and Shadow routers. It seems not so easy to find 3rd-party USB wifi adapters that work with OpenWRT.
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