In Asus website there is this screenshot:
https://www.asus.com/support/FAQ/1005714/
From it, I understand that Watch Dog is available with Load Balance in the original firmware.
Was it removed from RMerlin?
I haven't removed or changed anything related to Dual WAN. Note however that Asus is constantly making changes to the firmware, so FAQs often end up being outdated.
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