Network load balancing is the ability to balance traffic across two or more WAN links without using routing protocols like BGP.
This example explains how to use multiple gateways with one taking over when first fails. It begins with adding the gateways. Configure larger distance value for the secondary one, and check-gateway for the first one:
/ip route add gateway=192.168.1.1 distance=1 check-gateway=ping /ip route add gateway=192.168.2.1 distance=2
The first gateway will begin as it's distance is smaller (default 1); the check-gateway will make sure it is up. When the ping will fail, it will disable the first gateway and the second will take over and when the first one comes up, it will return to its function.