Hi,
This is a known problem, see: Archer C7 2.4 GHz wireless dies in 24~48 hours
Basically, 2.4GHz interface will start acting up if loaded too much. 5GHz is rock solid. For me, it was the same in 18.06, 19.07 and 19.07.1. replacing ath10k-ct with (newest version of) ath10k did not work either.
Currently, only thing that seems to work is to revert to older driver:
opkg install http://downloads.openwrt.org/releases/17.01.7/packages/mips_24kc/base/ath10k-firmware-qca988x_2017-01-11-ab432c60-1_mips_24kc.ipk