@eth Try this, it might work:
uci set network.wan.metric='10'
uci set network.wwan.metric='20'
uci commit
/etc/init.d/network restart
With this, default(0.0.0.0/0) will be routed to the WWAN only when the WAN link is down.
In simple terms, the WAN has priority over the WWAN.
You can adjust the numbers, or set some for other interfaces as well.
There's another recommendable one - mwan3
It provides much detailed options, and precisely detects the actual "online" status, based on the fine-tunable criteria.
Below is my actual use cases:
But it's temporarily unavailable now:
Try mwan3 when it back!
opkg update
opkg install luci-app-mwan3