iMac connected to WiFi but no internet

System Settings>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

1. On your Mac, choose Apple menu  > System Settings, then click Network  in the sidebar. (You may need to scroll down.)
2. 
   
3. Click the Action pop-up menu  on the right, then choose Set Service Order.
4. Drag services into the order you want.
5. Click OK.

The interface that connects to the Internet should be dragged to the top of the list.

System Settings>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Setttings>Network, click a network service on the right, click Details, then click DNS. (You may need to scroll down.)

Enter these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

What Mac is it? An iMac? Is it connected to your router via wired ethernet (a cable); or via WiFi?

Try turning off WiFi, then shut down your Mac. Restart your Mac and turn WiFi back on. See if it connects & gets an internet connection. Let us know what happens.

If you are using WiFi and it's not working, try connecting via an ethernet cable instead. (Turn off WiFi when you do this.) This would help diagnose whether the problem is in your Mac vs. in your WiFi router. Again, let us know what happens.

some wifi routers keep 2.4Ghz and 5Ghz as 2 separate unbridged networks, if the mac connects to another one than the rest of your devices and the one the mac connects to is not configured to access the internet it could explain what you experience