Using failover protocol alone will meet your needs.
Load Balancing simply lets you create rules to divide traffic ratio between different interfaces.
As you can see from the image above, the configuration is very simple. You can assign ratio values to WAN interfaces. The ratio values represent a percentage of load that will go through an interface. For example, in the default configuration 3 parts of traffic will go through the Mobile interface and 2 parts will go through the Wired interface, which means roughly 60% (3/5) of data will be transferred through Mobile, 40% (2/5) through Wired. If the ratios would be different, say Mobile: 5, Wired: 10, then 33% (5/15) of data would be transferred through Mobile, and 66% (10/15) would go through Wired.