i guess the 3b is an alternative to 3a, while 3a sets usb0 to your uplink, 3b sets two wan interfaces.
there is a package mwan3 that can be configured to switch between wan interfaces:
https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3
maybe you could use that one to automatically switch to mobille uplink when your provider fails.
Another example I wanted to try, is to use wi-fi tethering using mobile phone, you can check what I found so far (I’ll try to keep updated):