After spending weeks trying to connect my Apple Watch Series 5 to Xcode, I suddenly discovered that all watches before the Series 5 (including the Series 5) can only connect to 2.4G Wi-Fi.
My iPhone and Mac are connected to 5G Wi-Fi. So when I tried connecting my iPhone and Mac to the 2.4G Wi-Fi, the watch connected instantly.
But that's weird, There are a few times that I still using 5G Wi-Fi for iPhone and Mac, but I can still connect to my series 5. There might be some bug here for connecting watch on different Wi-Fi but actually the SAME LAN.