What is WAN Failover?
Failover enables you to connect a second Internet connection to your UniFi Gateway which will serve as a “backup”. If your primary Internet service goes down, you will begin utilizing your secondary Internet connection.
How does UniFi determine if my Internet goes down?
The UniFi Network Application checks for connectivity and latency to an “echo server”. By default, this is set to ping.ui.com which leverages responses from various locations to ensure maximum accuracy.
Note: Some advanced network administrators may choose to manually select their own echo server depending on their specific requirements.
What is WAN Load Balancing?
Unlike WAN Failover which only uses a single Internet source at a given time, WAN Load Balancing will split Internet traffic between both of your sources. This will be supported by UniFi Gateways beginning in version 1.13 (UDM Pro / UXG Pro) and 2.5 (UDM SE).
How many Internet connections are my UniFi Gateways capable of?
In addition to the two WAN connections, UniFi Gateways also support the use of our UniFi LTE Backup which is connected to a LAN port. This is only capable of being used as a failover option.
Source :
https://help.ui.com/hc/en-us/articles/360052548713-UniFi-Network-WAN-Failover-and-Load-Balancing