Chrome's probe found the network configuration itself unusable: the DNS servers your device was given are wrong, dead, or unreachable, typically handed out by a router whose own settings are stale, typed in manually long ago and forgotten, or left behind by a VPN. The internet may be physically fine; the instructions your device follows to use it are broken, which is what “bad config” means.
1.1.1.1 and 8.8.8.8 bypasses a broken router resolver entirely. If pages load immediately after, the router's DNS was the fault.ipconfig /flushdns. macOS: sudo dscacheutil -flushcache. Cached failures otherwise outlive the fix.If every device on the network reports bad config, the router is handing out the broken settings: log into it and correct its DNS entries (automatic from the provider, or the public resolvers above) so the fix reaches every device at once.
Why did this appear after my provider did maintenance? Providers retire and renumber DNS servers. Routers and devices holding the old addresses by manual setting or stale DHCP leases keep asking servers that no longer answer. Renewing the settings, or restarting the router, picks up the new ones.
Is using a public DNS like 1.1.1.1 safe? Yes; public resolvers from established operators are a standard diagnostic and a common permanent choice. It changes who answers your name lookups from your provider to that operator, nothing more, and it is reversible at any time.