Have the same problem. My ISP is big and not cooperating, the solution with modems local ips would put me through hassle with double NAT or making these crappy modems to route. My current GW (vigor 2820 with dual wan) have no problem with two wans with same gw, but i need to add more wans.
The thing is, to achieve best performance i need the modems just to bridge, spoof theyr mac address and route, balance and qos traffic eith something more efective/efficient like ZS. but after allnight hassle i came nowhere. And my provider doesnt use PPPoE therefor it just need an IP as a default gw with conjunction with interface.
so far i have easy testing setup, which perfectly simulkates the situation:
Local network gw: 10.10.1.1
ZS WANs to loadbalance: eth01: 10.10.1.213
(ZS complains about theese settings, so i just use DHCP to assighn WAN addreses. and yeas i know and agree that thats not clean setup, but tell the ISP, they even have 255.255.0.0 netmask, lame…)
zs lan: 10.0.0.1
its wery much the same setup like our isp have, but without all the dslams and so.