What should be tested before relying on dual-SIM failover at an unattended site?

Test both SIMs independently first. Confirm that each registers, receives the expected APN, passes traffic and can reach the services the site actually uses. Record signal-quality readings and data performance at the installed antenna position, not only on a workbench.

Then simulate failures: block the health-check target, remove the primary WAN, disable mobile data, create a weak-signal condition if practical and verify the router changes paths within the required time. Confirm alerts are received and that the router returns to the preferred service without repeatedly flapping between connections. Check what happens to VPNs, remote sessions, cameras and cloud links after the public IP changes. Finally, test remote recovery and document the configuration. A failover feature is not dependable until the whole application has been tested around it.