I have managed to set up a working back-up WAN via the USB port and got a tethered 4G phone working. However, after that WAN interface has been used and you unplug it, the normal WAN is not working – you need to do /etc/init.d/network restart to get it going.
What is the best way to solve this issue?
the mwan3 package ?
or perhaps a hotplug trigger.
Yes mwan3 configured for failover switching.
Although the most effective way to control multiple uplinks is mwan3 as mentioned before, a quick way to solve your issue is to assign different metrics to the uplinks. This way when you connect to the wan, its gateway entry won't replace the existing from tethering in the routing table. And in fact this is anyway the first step when setting up mwan3.