I have a different router (Linksys WRT1900AC) but also had low wifi speeds on 2.4Ghz only. Tried everything and finally resolved the issue by switching my 2.4Ghz network to channel 9. "Auto" wasn't giving me high speeds and neither were the standard 1, 6, and 11 channels. Since I live in an apartment building with heavy wifi congestion it seems like a non-standard channel in my case works best given my current environment. Not sure if this helps you but worth mentioning.