I can connect my Apple Watch to WiFi without using my iPhone

Is that a question? Are you trying to ask what you can do with the Apple Watch without its paired iPhone? It can only connect to wi-fi networks that you have connected to in the past with the iPhone, and cannot connect to captive wi-fi networks. How to use your Apple Watch without your iPhone nearby - Apple Support

Test this when you are at home and you have the phone with you. Make sure that your phone is connected to the wi-fi at home and then look at Control Center on the Watch to ensure it shows connected to wi-fi. Then power the phone off, and make sure the device then connects to wi-fi. If this works, then it may be the network that you are trying to connect to.

Try here > How to use your Apple Watch without your iPhone nearby - Apple Support