I found also what appears to be a very similar problem, on a different hardware and different OpenWrt version.
Noticing that, I connected the USB adapter to my Archer C7 V2 running also 19.07.0 and it seems to work (after installing the driver of course. So it seems the issue might be related to Lantiq xrx200.
From Archer C7 V2 log:
[ 16.130852] usb 2-1: ath9k_htc: Firmware ath9k_htc/htc_9271-1.4.0.fw requested
[ 16.138365] usbcore: registered new interface driver ath9k_htc
[ 16.242982] ath: EEPROM regdomain: 0x0
[ 16.242990] ath: EEPROM indicates default country code should be used
[ 16.242993] ath: doing EEPROM country->regdmn map search
[ 16.243006] ath: country maps to regdmn code: 0x3a
[ 16.243011] ath: Country alpha2 being used: US
[ 16.243014] ath: Regpair used: 0x3a
[ 16.260906] ieee80211 phy1: Selected rate control algorithm 'minstrel_ht'
[ 16.262349] ieee80211 phy1: Atheros AR9550 Rev:0 mem=0xb8100000, irq=13
[ 16.304133] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 16.711259] usb 2-1: ath9k_htc: Transferred FW: ath9k_htc/htc_9271-1.4.0.fw, size: 51008
[ 16.974030] ath9k_htc 2-1:1.0: ath9k_htc: HTC initialized with 33 credits
[ 17.449875] ath9k_htc 2-1:1.0: ath9k_htc: FW Version: 1.4
[ 17.455486] ath9k_htc 2-1:1.0: FW RMW support: On
[ 17.460263] ath: EEPROM regdomain: 0x0
[ 17.464065] ath: EEPROM indicates default country code should be used
[ 17.470610] ath: doing EEPROM country->regdmn map search
[ 17.476016] ath: country maps to regdmn code: 0x3a
[ 17.480874] ath: Country alpha2 being used: US
[ 17.485393] ath: Regpair used: 0x3a
[ 17.620525] ieee80211 phy2: Atheros AR9271 Rev:1
Comparing the log with the log from BTHH5A, it seems that things go wrong for BTHH5A at:
[ 17.180162] ath: phy2: Failed to read SREV register
[ 17.180188] ath: phy2: Could not read hardware revisions
[ 17.183626] ath: phy2: Unable to initialize hardware; initialization status: -122