How do you connect to the ISP?
I have similar setup and it’s working for Internet access, except that there is no load balancing. You can see my setup in the thread https://www.zeroshell.org/forum/viewtopic.php?t=4216
I have eth0 LAN and eth1/eth2 WAN.
eth0 is assigned a private block IP address.
eth1/eth2 have ppp0 and ppp1 connections on them. In my case both have fixed IP provided by the ISP, but it worked when previously I was on dynamic IP addresses.
I did not have to assign gateway. It just worked as soon as ppp connections were up.