My iMac 27 inch late 2014 can’t get on Wi-Fi

Moved into a new home, and had to get a Wi-Fi extender to get Wi-Fi working in my office. The extender covers the area just fine - under Network in Settings, the light is green, says “Connected”. But when you try to go online by launching a window (for instance), the screen says “You Are Not Connected To The Internet”. This problem is only with my iMac (running macOS Big Sur 11.6.6). “

My iphone (13 Pro Max) and iPad (ipad Pro), and laptop (MacBook Air) all connect to the Wi-Fi extender and the Wi-Fi works seamlessly. How do I connect the Desktop iMac to the internet?