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.

未指定文件或测试。复现 Ethernet-plus-low-signal-SIM 场景，然后定位处理 WAN 回程选择和调制解调器连接的 Lua 入口点；比较有 SIM 和无 SIM 时的流量行为。只要在调制解调器信号较弱的情况下，视频通话和播放仍能通过稳定的强 Ethernet 连接保持稳定，即视为完成。

由索引模型根据 Issue 内容生成。