There are mainly two ways to obtain what you want.
- Netwatch, here is the simplest approach possible:
Simpler Failover for two Gateways I found working - #19 by jaclaz - recursive routing (basically you use a known reliable site as "virtual" gateway), here is the simplest way to make a recursive route explained:
Simple recursive failover for bears of little brain
Since you are new, it is advised that you go through these, before anything else:
The twelve Rules of Mikrotik Club
GP & CSA (Good Practice and Common Sense Advice) for Mikrotik devices
Where? 