Sounds to me that you have probably two default/static route with differents administrative distance and the preference is through WAN2. So, when you try access using WAN2, it works. When you try access using WAN1, the router try to send traffic back using the WAN2 and fail. Which means, the traffic get in using WAN1 and try to get out using WAN2 and fail.
Then, when you disable WAN2, it send traffc back suing WAN1 and it works. Which means, the traffic get in using WAN1 and get out using WAN1
You can share you router config just to make sure I am right. Then, we can think how to overcome it.