I've been dealing with this same problem and I think I figured out what causes it, but don't have a solution. My conclusion is the dd-wrt firmware on wzr-hp-g300nh doesn't work well with docsis 3 cable modems.
I was having some problems with my ambit docsis 2 modem overheating, so I got it replaced with a ubee docsis 3 modem. Immediately I noticed my download speeds drop from 30Mbps to 12Mbps. There were absolutely no other changes to my network (upload speed didn't seem to be affected much). I called my ISP tech support after I had done all my usual troubleshooting, and of course he suggests plugging the modem directly into my computer. I completely dismissed him at first because I knew my network was setup properly. Sure enough, I got 30Mbps when I bypassed the router and had to appologize. I plugged my router back in and started getting 12Mbps again. He wasn't much help with MTUs or any other network settings, so I decide to keep digging on my own. At this point I know it's a communication issue between the modem and the router, so I backup my config and start messing with settings. First the ones I think may cause an issue: making sure no other devices are using bandwidth, reverting to stock settings, turning off the firewall, ensuring QoS is disabled, etc., and then everything else. Absolutely nothing helps, so I call tech support. The Buffalo rep says there are no settings in the router to cause this, other than the hardware may be going out, and he can setup an RMA. I didn't think that was it, but I was stumped. Incidently, while I was messing with settings, I had changed my wireless settings to turbo 20Mhz/40Mhz (apparently the Wii doesn't like that). During troubleshooting for *that* issue, I jumped on my laptop and did another speedtest to test if my wireless network was working. To my surprise, I'm getting 25Mbps download over wireless. So my LAN connections are only getting 12Mbps and my WLAN connections are getting 25Mbps (~30Mbps - overhead). I needed to do one final confirm, so I plugged in my old junky wrt54g, and it's also getting 30Mbps... on dd-wrt firmware no less. By process of elimination, all we're really left with is the dd-wrt firmware not working properly with docsis 3 modems. All-in-all, I really like this router, but this is a HUGE issue.
tl;dr: Buffalo needs to come out with an update for the pro firmware.
