I am wondering if there are some usable and compatible 3rd party N-type Wifi antenna for the APX 320X? I mean just one additional original Sophos antenna is almost half the price of the APX 320X. This is ridiculous, it´s just some plastic with a wire on a thread. Can I buy some cheap one instead and use?
The certificate of wireless technology is always made on the delivered hardware. See: https://docs.sophos.com/nsg/other/RegulatoryCompliance/en-us/index.html
This means, you could potentially void your status and get fined by that.