There r nothing much u can do since the biggest problem for 2.4G is interference, but I will try the following settings to maximize the coverage:

Use only 20MHz (20MHz has higher txpwr and lower interference)
Disable AX (MCS11 has lower txpwr UNLESS the 2.4G device u r using has AX supported, then enable AX might helps)
Try middle channels (normally channel 6 has the highest txpwr comparing to channel 1/11)
Use only MCS 7 in Modulation Scheme
Optimize the antennas position (this is kinda tricky, u should learn which antennas r broadcasting 2.4G, maybe is the front 4 or rear 4, for AC-RT5300 it's front 4 antennas. If u wanna cover multi-storey building, make it like "\||/", if same floor then make it like "||||", and try to make 4 of the 2.4G antennas face to the place which u wanna cover most, face to not point to)

These r the things I will try to do before adding an extra AP, if it doesn't help then get an extra AP is the only choice.