The topic of 2.4GHz Wi-Fi support on the Archer A9 has already been discussed ad nauseum: 2.4 ghz radio support for Tp Link Archer A9 1900
Basically, until support for the QCN5502 chip is upstreamed into the Linux kernel (as part of ath9k) and subsequently pulled into a future OpenWRT release, your options for 2.4GHz support are to a) revert to stock firmware or b) get a different device with better OpenWRT support. Or you could go for c) write your own kernel driver, but I'm guessing that's not a feasible option for someone who's new to Linux 