So I flashed 19.07 on to 18.06.04 with sysupgrade image changing firmware to ath79. After a while I spotted that my laptop is no longer connecting with 80mhz width. So I tried changing channels. Then my 5G wifi died. I flashed again 19.07 RC1 this time factory image. Still 5G wifi is dead. I set WPA3-SAE / WPA2-PSK mode. Used hostapd-openssl package.
EDIT
Also I tried switching "Allow legacy 802.11b rates" off for fixing channel width. If I recall correctly this with changing channel resulted in 5G dying.
I have uninstalled ct versions and installed "ath10k-firmware-qca988x" and "kmod-ath10k" packages. After reboot 5G waked up but still only 20 mhz channel. Where on 18.06 I had working 80mhz. My laptop has Intel AX200 card.
EDIT
I switched on Windows to WPA3 from WPA2. But I don't know if that is a reason cause I didn't checked channel width before that.
Still 20 mhz width after forcing. I've also checked it against speedtest if thats only wrong reading. But I get around 10Mbits on 600 Mbits fibre. So it is 20 width.
Changed country from US to correct one. Still 20. There is almost no 5G wifi in my area. So channels are free. I'm trying on 48 channel. From time to time I can see in LuCi 40 width for short moments. After that back to 20. Also I'm like 2 meters from router so it's not range problem.
Doesn't matter if there's no interference, sometimes DFS has to listen for a period of time before any broadcast can happen. Run dmesg or logread and you should see whether or not DFS is listening.
pdev param 0 not supported by firmware
and in logread Mon Nov 11 00:07:16 2019 daemon.notice hostapd: 20/40 MHz operation not permitted on channel pri=11 sec=7 based on overlapping BSSes
Seems like this combo is not allowed by your regulatory domain (PL). Maybe try another channel?