I have BTHH5A as modem + main router + WDS STA on the 2.4 GHz radio, C7 V2 as WDS client on a dedicated SSID, and working as AP serving both 2.4 and 5 GHz clients, plus wired clients. The C7 also served SMB and miniDLNA.
All working well on 19.07.3 until recently when both radios on the C7 started going off by themselves, along with their LEDs. I thought it was a glitch, but even recycling the power on the router didn't always fix the issue (and needless to say, the WiFi hardware button in this device is known to have issues with OpenWrt, so, unsurprisingly, switching it off and on didn't do much good.
Connecting to the router via a cable and opening LuCI, I found the radios disabled; I enabled them and they worked.
So after that issue happened a few times, I decided to upgrade to OpenWrt 19.07.4, and I also took the chance to clean the wireless config file off the settings of a USB WiFi adapter that I had previu9osly instaleld but not plugging it any more.
That seemed to work for a week or so, until it happened again today. I was setting next to the router (literally), so I'm sure that nobody touched the hardware button.
I will include the logs in the replies below.
Does it look like a hardware issue and that the router is simply getting near the end of its life?