Occasionally, you need more than one external network connections. If one outside (WAN, Internet, whatever you want to call it) fails, your router should automatically swap to a second.

OpnSense has done an excellent job of setting this up.

• https://docs.opnsense.org/manual/how-tos/multiwan.html
• https://docs.opnsense.org/manual/multiwan.html
• https://docs.opnsense.org/manual/gateways.html