I have been battling with this issue on my MacBook Pro 16-inch for several days now. It would connect to the wifi, but not internet. Even though other devices connected to the wifi could get online just fine. Tried everything from OS reinstall to zapping NVRAM, from deleting Wifi locations and preferred networks to rebooting and upgrading router firmware. Ran hardware diagnostics. Changed DNS servers. Forgot and rejoined the wifi. Did hard reboots. At best, the internet would stay on for a few minutes after reboots. As a last ditch resort I turned off my Apple bluetooth keyboard and mouse and... BAM, internet is back. Still had some sporadic drops, so I put my iPhone and Apple Watch on Airplane Mode, and shut down Bluetooth on my MacBook Pro. Now it seems that I am able to remain online. It doesn't make any sense technically to me, but since I see many desperate people here I thought I would share, even though it may be that correlation ≠ causality.