There are also TP-LINK TL-WN722N units (v2, v3 and v4) with a Realtek chipset that does not work with free software! So ask for confirmation that the unit you will buy is Atheros based or you risk ending with nonusable unit.

Plug and play.

To get my TP-LINK TL-WN722N V1 to work on Trisquel 6.0 Toutatis, I installed the latest Linux-libre from jxself's Freesh repository and installed the free software open-ath9k-htc firmware package (source is available here).

To use Wi-Fi with Tehnoetic TET-N150HGA High Gain Antenna and FSF-certified TET-N150 Mini adapters in Trisquel, I installed the free firmware from the official Trisquel package open-ath9k-htc-firmware. It is included in both Trisquel 7 and Trisquel 6 updates repositories.

sudo apt install open-ath9k-htc-firmware

sudo dpkg -i open-ath9k-htc-firmware.deb

sudo cp htc_9271.fw /lib/firmware # check if this path exists

sudo cp htc_9271.fw /firmware # check if this path exists

Bus 002 Device 003: ID 0cf3:9271 Atheros Communications, Inc. AR9271 802.11n
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass          255 Vendor Specific Class
  bDeviceSubClass       255 Vendor Specific Subclass
  bDeviceProtocol       255 Vendor Specific Protocol
  bMaxPacketSize0        64
  idVendor           0x0cf3 Atheros Communications, Inc.
  idProduct          0x9271 AR9271 802.11n
  bcdDevice            1.08
  iManufacturer          16 ATHEROS
  iProduct               32 USB2.0 WLAN
  iSerial                48 12345
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           60
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           6
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      0 
      bInterfaceProtocol      0 
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x83  EP 3 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x04  EP 4 OUT
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x05  EP 5 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x06  EP 6 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.00
  bDeviceClass          255 Vendor Specific Class
  bDeviceSubClass       255 Vendor Specific Subclass
  bDeviceProtocol       255 Vendor Specific Protocol
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0000
  (Bus Powered)

  UDI: /org/freedesktop/Hal/devices/usb_device_cf3_9271_12345_if0
  Unique ID: lfzD.7OI4S6n4RF2
  Parent ID: ADDn.4Nx_qoDfSd7
  SysFS ID: /devices/pci0000:00/0000:00:1a.0/usb1/1-1/1-1.1/1-1.1:1.0
  SysFS BusID: 1-1.1:1.0
  Hardware Class: unknown
  Model: "Atheros USB2.0 WLAN"
  Hotplug: USB
  Vendor: usb 0x0cf3 "Atheros Communications, Inc."
  Device: usb 0x9271 "USB2.0 WLAN"
  Revision: "1.08"
  Serial ID: "12345"
  Speed: 480 Mbps
  Module Alias: "usb:v0CF3p9271d0108dcFFdscFFdpFFicFFisc00ip00"
  Driver Info #0:
    Driver Status: ath9k_htc is active
    Driver Activation Cmd: "modprobe ath9k_htc"
  Config Status: cfg=new, avail=yes, need=no, active=unknown

03: USB 00.0: 0200 Ethernet controller
  [Created at usb.122]
  Unique ID: Uc5H.fcq4MaBNaZ4
  Parent ID: k4bc.2DFUsyrieMD
  SysFS ID: /devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.0
  SysFS BusID: 1-4:1.0
  Hardware Class: network
  Model: "Atheros AR9271 802.11n"
  Hotplug: USB
  Vendor: usb 0x0cf3 "Atheros Communications, Inc."
  Device: usb 0x9271 "AR9271 802.11n"
  Revision: "1.08"
  Serial ID: "12345"
  Driver: "ath9k_htc"
  Driver Modules: "ath9k_htc"
  Device File: wlxec086b130d7f
  Speed: 480 Mbps
  HW Address: ec:08:6b:13:0d:7f
  Link detected: yes
  Module Alias: "usb:v0CF3p9271d0108dcFFdscFFdpFFicFFisc00ip00in00"
  Driver Info #0:
    Driver Status: ath9k_htc is active
    Driver Activation Cmd: "modprobe ath9k_htc"
  Config Status: cfg=new, avail=yes, need=no, active=unknown
  Attached to: #2 (Hub)