I have my RT-AC3200 running link aggregation using Merlin's 56_2 release with no problems. This release has the bonding.ko module compiled and available as a loadable module. There are several posts about LAG on the site. I'd start with the Wiki and look for link aggregation there.
There is an updated script that can be found at https://github.com/necross2/asuswrt-merlin that has some updates for the RT-AC3200 port mappings.
I found that the port mappings on the router are reversed. For example, if I specify ports 3 and 4 in the parameters for the link aggregation script, I have to have the cable plugged into ports labeled 1 and 2. There was an earlier post (I don't have the link handy) that discovered that the physical port mapping are different from the labels.
Hope this helps.
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