If the maximum setting of the chip is 20dB, then technically no.
This will increase gain, perhaps. You can also buy an amplifier (or, a better router).
If you're measuring this with a power/spectrum meter (or looking the the Wireless page on LuCI), please recall newer devices use 802.11g, which is Spread Spectrum - they don't need as much power...how is your speed??
If you have the Web GUI, it's under Interfaces. You enter the starting number, and the number of leases.
EDIT: It could also be that you're using multiple SSIDs.
TX dB boost on all WRT AC Series routers manufactured in 2017> must conform to the FCC ruling that took affect in 2017, of which locks the transmit power above a certain amount (likely 20dB, but I can't recall for sure... I'd double check on google, or use Google's Advanced search to search the now dead WRT1900AC thread on the OpenWrt forum)
I believe this affects all WRT AC Series v2's, with the exception of the WRT1900AC v2.