Linksys EA6900 5ghz interface stuck at n speeds

I also have another issue which is probably related. 'nvram erase' does not clear all the nvram. After doing so, it shows 64/30 free (the 30 being the amount used). I think a lot of the nvram values that aren't being cleared are from DD-WRT.

Any way to fix this and get ac working again?

edit: I just realized the hardware is the same as an AC68U. Is it possible to cross-flash the router by flashing the bootloader?