Raspberry pi with two wlan and if eth is down, switch to wifif


I have a raspberry pi configured with openwrt and I am currently using the internal wifi as access point that connect to openvpn and that routes via eth0 (lan), used the following documentation (https://kiljan.org/2021/12/14/vpn-as-wan-for-guest-network-on-openwrt-2021-edition/#create-a-guest-network)

Now what I want to achieve is, with an extra wifi adapter (that is already installed), when eth0 (lan) is down, that it routes data including openvpn automatically to the extra wifi adapter that is connected to a 4g modem router.

How can I achieve this?

Looks like you need to use the "mwan3" package.