Is the issue replicated on the wired / LAN connection or only wireless?
What devices do you have connected to the 2.4Ghz wifi?
When your speed testing on 2.4Ghz, what device(s) are you using? Have you tested different devices on 2.4Ghz to see any differences?
When speed testing, are you disabling ALL other devices on 2.4Ghz and only testing with one connected device?
Can you do a wifi can to see what neighbors are near by and how many neighbors are near by on 2.Ghz? What channels are they using and how many?