If you want truly redundant, without losing connection when one ISP drops, you need to look at “internet bonding” which is essentially using VPNs to connect each of your connections to a data center where they are combined and go out to the open web. I use Speedify service in my (raspberry pi) router to do this. You can set different priorities of connections, and even set them to always send every packet through every connection if you have no data caps.
If you want truly redundant, without losing connection when one ISP drops, you need to look at “internet bonding” which is essentially using VPNs to connect each of your connections to a data center where they are combined and go out to the open web. I use Speedify service in my (raspberry pi) router to do this. You can set different priorities of connections, and even set them to always send every packet through every connection if you have no data caps.