Are you running a mostly default configuration? Have you installed additional packages? Did you keep settings when you upgraded from 18.06 > 19.07, or did you re-create them manually? Is there a pattern to the reboots (i.e. time, heavy load, a specific feature/function being utilized, etc.)?
One of my two devices had similar issues some time ago...
I switched the power units of them and then the other device was suffering from random reboots.
Can you test another power plug?
It's pretty plain. No additional packages. I recreated the settings manually when upgrading from 18.06 > 19.07. I haven't seen a pattern. The ISP is Comcast, so it's dual stack (ipv4 & ipv6).
I have same hardware, path, same isp, but reboots were daily at least, sometimes more. Main difference: I have a 3600 connecting as WDS client. It also was cross-graded to Ath79. 3600 is rock solid, even after swapping power supplies.
I just swapped the 4300 back to ar71xx, we shall see.
when I watch 4K videos it restarts in a couple of minutes, but sometimes I can watch two 4K videos for hours and nothing happens. No logs are being recorded, just simple restart, and it connects as a AP Client again, and I have Internet as before.
I was hoping to install OpenWrt instead of DD-WRT, but as I see, it's probably the same problem.
I've had the same issue with Cisco 1500 which restarted after watching 4K videos from 5 to 20 minutes time, also with DD-WRT installed. Only happens when using AP Client to connect to Wi-Fi in other room.
Also, switched for 3 power supplies (Original 12V 1.5, second 12V 2.5A and other 2A) and the issue is the same.