From time to time (a few days usually) my wireless network drops as in there is no longer a visible SSID and all the clients disconnect. (wlan0 goes down, disable+enable fixes it without a reboot). The device is set up as an AP, so there's no heavy load on the CPU. This is what I find in the kernel log after it happens and i re-enable it (system log shows nothing other than clients getting forcefully disconnected):
[91217.740095] ath: phy0: Unable to reset channel, reset status -5
[96278.937949] ath: phy0: DMA failed to stop in 10 ms AR_CR=0x00000024 AR_DIAG_SW=0x02100020 DMADBG_7=0x00028800
[96617.802860] ath: phy0: Unable to reset channel, reset status -5
[102017.866752] ath: phy0: Unable to reset channel, reset status -5
[103817.956163] ath: phy0: Unable to reset channel, reset status -5
[117617.891702] ath: phy0: Unable to reset channel, reset status -5
[123618.020150] ath: phy0: Unable to reset channel, reset status -5
[209418.259713] ath: phy0: Unable to reset channel, reset status -5
[313818.642144] ath: phy0: Unable to reset channel, reset status -5
[480019.158664] ath: phy0: Unable to reset channel, reset status -5
[483769.788128] device wlan0 left promiscuous mode
[483769.792867] br-lan: port 3(wlan0) entered disabled state
[483775.331732] IPv6: ADDRCONF(NETDEV_UP): wlan0: link is not ready
[483775.379709] device wlan0 entered promiscuous mode
[483775.737607] IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
[483775.744322] br-lan: port 3(wlan0) entered forwarding state
[483775.750005] br-lan: port 3(wlan0) entered forwarding state
[483777.741440] br-lan: port 3(wlan0) entered forwarding state
[489176.038274] ath: phy0: Unable to reset channel, reset status -5
[489775.998453] ath: phy0: Unable to reset channel, reset status -5
[493975.934999] ath: phy0: Unable to reset channel, reset status -5
I have exactly the same behavior since months (it got messed up with another issue, related to IPv6, so that I didn't realize that I had two issues, until a friend of mine reported a similar behavior).
Model: TP-Link TL-WDR4300 v1
Architecture: Atheros AR9344 rev 2
Firmware Version: OpenWrt 19.07.5 r11257-5090152ae3 / LuCI openwrt-19.07 branch git-20.341.57626-51f55b5
It appeared at least with 19.07.4, I still need to see it appear with 19.07.5. Did you solve the issue somehow? Any advice on how to debug the issue if it appears again?
# logread | grep DMA
Sat Dec 12 15:17:59 2020 kern.err kernel: [28085.293377] ath: phy0: DMA failed to stop in 10 ms AR_CR=0x00000024 AR_DIAG_SW=0x02100020 DMADBG_7=0x00026800
Sat Dec 12 15:18:30 2020 kern.err kernel: [28116.218313] ath: phy0: DMA failed to stop in 10 ms AR_CR=0x00000024 AR_DIAG_SW=0x02100020 DMADBG_7=0x00006400
Sat Dec 12 15:18:45 2020 kern.err kernel: [28130.861484] ath: phy0: DMA failed to stop in 10 ms AR_CR=0x00000024 AR_DIAG_SW=0x02100020 DMADBG_7=0x00026800
(sorry, pressed too fast the send button)
Woohoo, there's been quite some time since I posted this. Almost 3 and a half years to be more precise 
In my case, I think the problem was related to overheating, (I had more devices stacked under and over this one). After I gave it some breathing room the problems stopped.
This might not be related to your problem, but this was my experience.
Other unrelated "paranormal" experiences include the fact that having devices connected through USB 3.0 near the router seems to interfere with the 2.4 GHz band (specifically channel 1 in my case?), making it look like the wifi temporarily dropped. I later found out there's a whitepaper by Intel studying this phenomenon, so it's nothing new, I just had not heard of it. I just felt like sharing it with you since there was no-one telling me this when I was debugging my problems. 
Good luck!