I know you don't read my posts, but I'll still put this out there.
There are commercial products that work, but I would recommend a homebrew
solution based on pfSense. You can set up dual WAN connections, each to its
own ISP, where both ISPs are used as long as both are available, but if
either ISP goes down pfSense will automatically use the one remaining ISP.
Each ISP is monitored, via ping to a WAN IP, so if ping is successful the
ISP is assumed to be available.
Note that, due to the nature of TCP connections, WAN failover won't always
be entirely seamless, so expect to reestablish a connection in some cases.
To get started, you need a PC with 3 NICs, two for WAN and one for LAN, and
the free pfSense software. One example of the dual-WAN configuration is
TIP: You can configure "dual-WAN + 1 LAN" with just two NICS or even with
just one NIC, but the config guide above is written with 3 NICs in mind.
Separate NICs for each of the 3 networks will be easier to set up.