The easiest way is to configure one floating static route so that the link becomes active in case the first link...
Ip route 0.0.0.0 0.0.0.0 ISP_A
Ip route 0.0.0.0 0.0.0. ISP_B 50
Traffic will go to the ISP B only when the ISP A link is down. The drawback in this case is that you would be using only one link.
Another way is to have routing protocol and BGP so that you can utilize both the links almost 50% for the traffic going out of your network.
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