I reported this problem previously with 380.69. I tried 389.6_2 like Pat57 in the following thread and the issue persists for me too:
https://www.snbforums.com/threads/rt-n66u-loses-connection-with-latest-firmware-380-69.43042/page-2
The RT-N66U connects fine as a media bridge but then loses the connection and has to be rebooted after about 12-24 hours.
I've gone back to 380.68_4 on my RT-N66U. I use the RT-N66U as a media bridge and an RT-AC87U which runs 380.69_2 fine as the main router.
https://www.snbforums.com/threads/rt-n66u-loses-connection-with-latest-firmware-380-69.43042/page-2
The RT-N66U connects fine as a media bridge but then loses the connection and has to be rebooted after about 12-24 hours.
I've gone back to 380.68_4 on my RT-N66U. I use the RT-N66U as a media bridge and an RT-AC87U which runs 380.69_2 fine as the main router.