I went into 5G Wifi settings and I switched off "enable 160 MHz" and changed the Control Channel to AUTO which had been 40 before (based on some testing I did months ago). I'm not sure if this is what did it, but so far I haven't had any 5G disconnects (after about an hour).
I'm going to roll back to 384.9579 I think. Ever since I updated to 386.41249 my 5G has been cutting out. ALL of my 5G clients will drop at the same time. ALL 2.4 clients remain connected.
I am having a similar problem, although my connection comes back after 1-2 minutes. As far as I know it wasn't happening prior to the firmware 3.0.0.4.384_9579. When it has happened, I've checked my modem status using my ISPs app over my phone's cell network and it reports my modem is online, so...