First step configure Trunk
Make your own rule and use appropriate algorithm.
I use Spillover and only when the primary is saturated max settings ie never.
Its because I only use wan 2 for old email accounts and its very tiny aka throughput.
Whats important in the trunk rule is order, so the primary ISP has to be first.
Second go to policy route.
you have to make a rule that is first in order that sends that IP to WAN2
The rule after that should be general rule that says send it out as per the trunk.
See the attached example.......--
Ain't nuthin but the blues! "Albert Collins".
Leave your troubles at the door! "Pepe Peregil" De Sevilla. Just Don't Wifi without WPA, "Yul Brenner"