When the main link drops, commonly the alternate link is used if set up correctly. Install redundant UPS power and cooling units. Testing however is the key to your success.
If not, then you need an after-action report where you can specify how things will be fixed, or redesigned — then retested. Although highly available internally, if the router that connects your remote site fails, or the link fails… your site will be down regardless unless you have a redundant router and link there to pick up in the failed routers place.
You can connect a modem to the router, a server or laptop and have LAN access to troubleshoot or provide access out if needed.
However, detecting failures happens exclusively at the physical layer: the track features is not able to detect a failure occurred on a farther link beyond a bridge. Competing business rivals looking for your business will hope that you budget, plan and design this solution incorrectly.
It is not recommended that you use this particular Internet access to connect your VPN tunnels for backup, because it could be potentially connecting to the same network that failed you in the first place.
The track feature works only if the preemption capability is on: if the priority of the active router is decreased so as to bring it below the priority of the stand-by router, the latter can 'preempt' the active state from the active router by sending a Hello message of Coup type.
Network hardware failure is one obvious issue, but the systems surrounding your network also need to factor into your assessment: a short-circuit or power spike, HVAC issues, fire or flooding can also cause network downtime.
This gives the chance to test internally to the site from the local area network [LAN] in times of complete cut off. If you put in a secondary or tertiary method of access, then when the main connection goes down, you will have a way to connect to resources and keep the business operational. A large carrier, such as a telco, can also do this for you.
The main link failing means that the backup should take over automatically if that is how you designed it.