If you mean the SSID is using NAT mode that means clients get 10.x.x.x/8 IPs and NAT to the mgmt IP of the AP they're connected to. So, the MX wouldn't see the client as a 10.x.x.x/8 IP but rather whatever the mgmt IP of the AP is.
If you're just trying to load balance outbound traffic just set the uplink load balancing to enabled. If you're trying to ping certain wireless traffic to WAN2 you should consider using bridge mode so the MX sees the real client source IP.
Ryan If you found this post helpful, please give it
Kudos. If my answer solves your problem please click
Accept as Solution so others can benefit from it.