@simonJ Follow the 'getting connected' guide, specifically here: https://duet3d.dozuki.com/Guide/1.)+Getting+Connected+to+your+Duet/7?lang=en#s9
Usually, I find it more reliable to turn off WiFi with M552 S-1, then turn it on in idle mode with M552 S0 before adding SSID, if there's no existing WiFi connection. Then send M552 S1 to enable WiFi.
Ian