Sorry about the late reply.
It sounds like you're not changing the outgoing rule. You need both the NAT rule (or just use AoN) and a firewall rule on the LAN interface that sends matching traffic out your second WAN. The NAT rules are just there to perform NAT on traffic leaving those interfaces coming from the specific networks, while you need the firewall rules to actually direct the traffic there in the first place.
If you want to load balance you need to set up the round robin pool in the load balancer and use that as your the 'gateway' in your firewall rule instead of one of the WANs, but I'd suggest that you get it set up such that you can route traffic manually out either interface and that's working properly before you try and get load balancing/failover going.