Since your Odoo website works correctly on browsers, but the /web route is unreachable from a mobile app, the root cause could involve URL routing, proxy configuration, or server-side limitations.

Here are some troubleshooting steps:

1. Ensure correct routing and proxy passing for /web in your NGINX config.
2. Confirm mobile app and browser are using the same HTTP/HTTPS scheme.
3. Check NGINX logs for more detailed information on the error.
4. Verify Odoo access rules for the /web route.
5. Add CORS headers to NGINX if cross-origin requests are involved.

Let me know if it helps.