Connection error on iPhone

### Describe the Bug

When connecting trough LTE connection the pangolin client shows a popup with the following text when moving the slider away disconnected. Disconnected changes to Registering. After a while the following error appears:

_Connection Error
Unable to coordinate client P2P connection. Please ensure your client can reach the server on UDP Port 21820 and try to registering again._

With WIFI connection the client connects as expected.

### Environment

- OS Type & Version: IOS26.5
- Pangolin Version: 1.19.2
- Edition (Community or Enterprise): Community
- Gerbil Version: 1.4.2
- Traefik Version: 3.4.0
- Newt Version: 1.13.0
- Client Version: Pangolin App 0.9.0 (23)

### To Reproduce

Try to connect via mobile data and telekom.de as provider.

### Expected Behavior

Connection is also possible via mobile data transfer.

Start by reproducing the issue in the Pangolin App on an iPhone over Telekom LTE, using the listed app and client versions, and trace the mobile-data connection attempt around UDP port 21820. Done means the client can connect over mobile data as it already does over Wi-Fi, without the connection error.

Written by the indexing model from the issue text.