I am struggling with this router, too. I did get the hotspot to work for a short period, but then I moved it to a new site and am back to working with the Hotspotsystem support. Anyway, some info on the build that I finally got working:
dd-wrt.v24-22118_NEWD-2_K3.x-big-RT-N16.trx – HOWEVER, I bricked my router twice during firmware updates before reading that you have to start with a K2.x build, then move to K3.x 21530, and finally to 22118 for it to work, otherwise it soft-bricks. More info can be found here, 7 posts from the top: https://dd-wrt.com/phpBB2/viewtopic.php?t=174671&start=30&sid=f4bf41a46f5eadd4e002f7cc9155465e
So, I would recommend the approach above and see if that works. At a minimum, you will have a supported build installed so hotspotsystem support should be able to assist.