DSL (pppoe) / Cablemodem how to load balance doing NAT

Hi please I would like to know how to apply load balance I got the following connection scheme- Primary DSL (PPPoE Connection) with NAT , internal LAN clients assigned IP address through DHCP

  • Secondary Cablemodem Connection

I’ve tried to do load balance with the following syntax

/ip route add gateway=ip_address_1, ip_address_2

now problem is once I add to default gateway the IP address of the cablemodem the Public Address assigned to my cable modem , connection it’s not working anymore. Do I have to add another rule to make NAT work with cablemodem , I need a more detailed explanation on how to achieve this .

Thanks in advance.