Couple things to check:
Make sure you have at least one DNS server per WAN under System>General Setup if you're using the DNS forwarder (default out of the box config you are). Validate whether it's just DNS that's failing (you can't resolve names, but can ping things like 8.8.8.8), or IP connectivity fails. I suspect just the former because your DNS config isn't valid for multi-WAN. In general, I would not allow overriding your manually configured DNS servers with ISP-assigned ones when using multi-WAN.
Check your gateways status under Status>Gateways and make sure a failure is being properly detected. If it's not, it's probably because you're doing something like using your modem's IP for your monitor IP and it never goes down when your connection goes down.