Shortcuts 'Call Service' returns 404 on cellular, works on WiFi

**iOS device model, version and app version**

Model Name: iPhone 16
Software Version: iOS 26.3.0
App version: 2026.1.1 (build 1652)

**Home Assistant Core Version**

2026.2.1

**Describe the bug**

iOS Shortcuts using the "Call Service" action fail with a 404 error when triggered on cellular (5G). The same Shortcuts work correctly on WiFi. This started recently, likely coinciding with the HA Core 2026.2 update.

The error message displayed is:
> "Failed: Error during api.callService: Unacceptable status code 404."

The Home Assistant app itself works fine on cellular — WebSocket connects, dashboard loads, entities are accessible. Only the Shortcuts integration is broken on cellular.

Nginx access logs show webhook POST requests when Shortcuts are triggered on WiFi, but **zero requests reach the server** when triggered on cellular. This suggests the Shortcuts/Intents extension is either not sending the request or sending it to the wrong destination when off WiFi.

The external URL is confirmed reachable from cellular — Safari loads the HA login page, and the app itself connects via WebSocket and shows "Connected via: External URL" with "WebSocket: Connected".

**To Reproduce**

1. Configure HA companion app with only an external URL (no internal URL set)
2. Create an iOS Shortcut using the HA "Call Service" action (e.g., `lock.unlock`)
3. Run the Shortcut while connected to WiFi → works (200/201 in server logs)
4. Switch to cellular (5G) and run the same Shortcut → fails with 404
5. Confirm the HA app itself works on cellular (WebSocket connected, dashboard loads)

**Expected behavior**

Shortcuts should work on both WiFi and cellular, as they did prior to the recent updates.

**Additional context**

- Internal URL: not configured (set to `—`)
- External URL: configured and working via HTTPS
- Reverse proxy: nginx
- Connection security level: Most secure
- iCloud Private Relay: disabled
- Port 443 confirmed open externally
- The key observation is that **no HTTP requests reach the server at all** when the Shortcut runs on cellular — this is not a server-side 404, it appears the Shortcuts extension fails to dispatch the request when off WiFi