Connectivity is choppy when connected over a fast ETH WAN and a weak signal modem

## Test
Connect over Ethernet backhaul with a fast connection (I used 100mbps).
Insert a SIM under low signal strength conditions (I had 1 or 2 bars).
Try a video call or watch a video, there are a lot of drop outs.

Remove the SIM and reboot
Try a video call or watch a video, the experience is a lot better.

## Expected
Using multiple forms of backhaul, where one of them has a strong connection with high bandwidth, the user experience should be strong regardless of the quality of the other connections.

未指定檔案或測試。重現 Ethernet-plus-low-signal-SIM 情境，然後找出處理 WAN 回程選擇與數據機連線的 Lua 進入點；比較有 SIM 與沒有 SIM 時的流量行為。在數據機訊號微弱的情況下，視訊通話與播放仍能透過穩定的強 Ethernet 連線保持穩定，即表示完成。

由索引模型根據 Issue 內容生成。