Thank you for your question.
From document you've provided i can see 2 mistakes that you did.
1. In your interfaces WANB general settings you've chosen Static protocol, try changing it to DHCP.
2. In your interfaces WANB Physical settings you've attached two interfaces, leave only eth0.2.
Also here is updated Wiki article on how to setup LAN as WAN: https://wiki.teltonika-networks.com/view/LAN_as_WAN_RutOS
When you will create correctly second WAN it will automatically appear in Failover page.
Let me know if this helps you.