A JSON rest_no_route response means WordPress did not match a REST endpoint for that request. Check the route and HTTP method together. A web-server HTML 404 is a different failure and needs a different investigation.
Identify what returned the 404
Inspect the response body and content type. WordPress REST errors are typically JSON, whereas a server, CDN or security layer may return its own HTML error page. Record the request URL and method. A browser address-bar test sends GET, which does not test an endpoint intended for POST.
Verify the endpoint contract
Compare the client request with the integration documentation: namespace, version, route segments and allowed method. Check spelling and whether the client accidentally uses a duplicated path prefix after a migration. Do not guess a new route from the name of a WordPress page; REST endpoints are registered separately.
Check who registers the route
For a plugin endpoint, confirm the expected plugin and its dependencies are active and compatible on the affected environment. For custom code, inspect how the route is registered during REST initialization. The official REST handbook explains that route matching includes methods and that endpoints can define permission callbacks. Authentication failures should not be treated as permission to remove those callbacks.
Compare environments safely
If staging works, compare plugin versions, active components, site URL configuration and request routing. Use an equivalent test account with the required permissions. Do not paste API credentials into a browser URL or public support message. Rewriting permalinks may address a server routing issue, but it will not create a plugin endpoint that was never registered.
Verify the intended consumer
After repair, test the actual editor, form or integration that uses the route. Confirm the expected successful response and appropriate rejection of an unauthorized request. Keep a sanitized request example and note the component responsible for the endpoint, so a future plugin replacement does not silently break the client again.
Questions about this problem
Is every REST 404 a permalink problem?
No. A missing route, wrong method or inactive plugin can return a REST 404 even when ordinary page URLs work.
Should I disable REST authentication?
No. Verify the endpoint and its documented permission requirements. Removing authentication can expose site functions or data.
Primary references
General diagnostic guidance, not a diagnosis of an individual site. Use a recoverable copy for changes that may affect production.