Like @fellwalker57, I had internet problems (exactly mirroring those described by @toxdoc) after upgrading (in my case from Big Sur to Monterey) while using the Cloudflare WARP 1.1.1.1 VPN. After much frustration, I remembered that it was installed, turned it off, and all troubles were resolved.
Taking a page from @neil1, I just now created a new Network Location, manually changed my DNS from that provided by my router (which ought to be 1.1.1.1, but who knows for sure!) to 1.1.1.1, and turned Cloudflare’s WARP back on. No internet problems at all!
So, not entirely a scientific A/B test, but it looks as if WARP and the OS together can get confused if a DNS server other than Cloudflare’s 1.1.1.1 is used…