2 ISP Problem. Pls Help

Hello everyone.
I used to have only one ISP. A Cable Internet Connection with a
cable modem connected to a router, and the router connected to a 24
port switch. And the switch connected to about 20 computers.

Since the router is dual wan, i decided to add a second ISP(DSL)
as backup in case the other one is down. I planned to plug in the 2nd
ISP to the 2nd WAN port of the router in order to utilize the load
balancing feature of the router. And also I expected that the router
would automatically switch between the two ISP's, whichever is
available. Unfortunately, the 2nd ISP provided me with a modem/router.
They wouldnt allow me to configure the modem/router as a router only so
that i can plug it to 2nd WAN Port of my existing router.

Can I connect the modem/router to the 2nd WAN port of my existing
router without reconfiguring the modem/router as a modem only? Is there
any other way i can configure my setup so that the 20 computers can
access the internet as long as at least 1 ISP is available?

Any help would be highly appreciated. Thanks in advance.



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