As others have pointed out, you need a dongle that is supported by free software and thus Trisquel.
There's a workaround. You can get a router that has wifi and an Ethernet port.
https://www.tp-link.com/lk/products/details/TL-MR3020.htmlThis router most likely has non-free firmware. But it works just like an external wifi adapter without installing non-free drivers in your Trisquel.
