The web UI is limited on the SRX and many scenarios are not covered.
This example of dual ISP with failover should give you what you need for the dual routing instances, security and nat policies.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB15545
I've not use pppoe on the SRX so I'm not sure what the support is there for various interfaces but this is the basic example.
https://kb.juniper.net/InfoCenter/index?page=content&id=kb15736