Load balance will work fine on 2 DHCP connections.
Add below , so load balance uses fixed remote IP to test connectivity, instead of relying on DNS
configure set load-balance group G interface eth0 route-test type ping target 8.8.8.8 set load-balance group G interface eth1 route-test type ping target 8.8.8.8 commit save exit
Since you're using failover mode, portmappings should also work. (on active WAN link)
Use this as starting point: KB