I have a WRT1900AC v1 running 18.06.1.
Why does it take the 5GHz WiFi so long to come up after a reboot?
The 2.4 GHz comes up fairly quickly, but the 5GHz WiFi takes an additional 4 or 5 minutes before it is operational.
I have also found that if I make any changes to the 5GHz WiFi configuration, after the changes are saved and applied, the 5GHz WiFi will not come back up again until I reboot the router.
If you are using a DFS channel, it will take 60 seconds before it will come up (if it comes up at all).
Ok, I knew nothing about DFS channels until now. I researched DFS and it is an interesting technology.
I did find that the Openwrt WiFi defaults to channels set to 'auto', so I set both WiFi to specific channels and the WiFi comes up much faster. I think I will set the 5 GHz to a non-DFS channel and see if it further improves.
I really appreciate the quick answer. You helped me learn something today!