No connectivity when switching to backup ISP

So, Multipath wasn’t designed for this exact use case (more so where you have two active links presented to ZT), but you could try using active-backup in your local.conf like so:

{
  "settings":
  {
    "defaultBondingPolicy": "custom-active-backup",
    "policies":
    {
      "custom-active-backup":
      {
        "basePolicy": "active-backup",
        "failoverInterval": 10000
      }
    }
  }
}

This will increase ambient traffic (to measure links) but it should cause a faster switch to your new link.

Let me know if you have any other questions on this.

EDIT: Fixed typo in example