Just had a really odd problem which I've managed to fix luckily.
ASUS DSL-AC68U which has always been perfectly reliable.
However woke up to no internet this morning even though DSL connection was up. BT couldn't see any problems so booked an engineer 2 weeks out.
Whilst having a good poke around in the router logs and settings, I could see that the firmware was updated automatically at 2am.
And somehow the update manged to wipe out the username and password.
Once I had put that back in as firstname.lastname@example.org and BT, happy days and everything was back to normal.
Thanks Asus! Doesn't seem to be a setting to turn off auto updates.
I also experienced this problem many many months ago.
Thankfully I had a backup router and managed to fault find from that,
It seems that with the most recent update that I applied - manually - that it now checks daily for new software but as yet not seen it do an automatic update.
I might go back to a previous version if it's now being done automatically as that's not what I really want to happen.
Looks like this auto update feature might have been included in Version 184.108.40.206.386_38287 , released June 30. No mention of it in the release notes specifically , just a mention of "Fixed live update issues."
Mine actually updated a couple of days ago when I scrolled through the log to Version 220.127.116.11.386_39648. Bloody cheeky of Asus I think and unfortunately they closed their user forum quite some time ago.