Just got back from the dd-wrt community forum and now I remember why I moved away from dd-wrt in the past (to other fork like tomato). It's a pain to understand what to flash, all documentation are badly maintained, etc... According to what I found, I must flash something from 2008 as the most stable for ac3100. Really, since 2008, no stable firmware? They say to not use the router database because it give unstable build. It's such a shirt show compared to merlin...
All firmwares have their issues. Years ago DD-WRT made a fateful decision to support virtually every consumer-grade router that came down the pike, regardless of chipset (Broadcom (where it all started), Atheros, Qualcomm, Marvel, Ralink, etc., even x86!). As a result, what limited resources were available were spent supporting this monumental effort, and unfortunately at the cost of everything else, including documentation, and even keeping the database itself up to date. Firmwares like Merlin and FreshTomato have the luxury of only needing to support a tiny fraction of those same routers, and all of them Broadcom. And in the case of Merlin, it's even based on the same source as the OEM. So despite DD-WRT's shortcomings, if you have anything other than Broadcom, it's usually your only other viable option.
All that said, w/ the RT-AC3100 being Broadcom, doesn't seem to me to be all that complicated a process to upgrade.
And the latest build from
@brainslayer is available from the following location.
Now granted, like any firmware, if you're NOT familiar w/ it, it's going to take time to learn it. And it's not like you're going to get a lot of hand holding. But having to use Merlin and work from the CLI to deal w/ VLANs is going to leave you in much the same position. Esp. given you're using AP mode, where many things change compared to the default router mode.
So your choice. Again, if it was *me*, it's a non brainer. DD-WRT is going to make things easier in the long run. That's from someone already familar w/ DD-WRT, FreshTomato, and Merlin, and who realizes the challenges ahead of you.