I've got a DIR-850L rev a1 which I've had for a couple of years. I use it with a speedstream dsl modem.
Suddenly connected devices can no longer access the internet when the DHCP server is enabled. Windows 10 for example will say it can't find DNS.
I hadn't changed anything to cause this, and the router status page looked the same as usual.
I tried resetting the router, upgrading from fw1.12 to fw1.14, no change.
It actually started exhibiting thus behavior a few weeks ago, except on those occasions, it would start working again before I could troubleshoot it this far.
Does it sound like the router is giving up the ghost?