I am having exactly the same problem as OP. It's driving me bonkers.
Symptoms are exactly the same, router works perfectly fine, then kicks off a device and the device will not be able to successfully reconnect. A little while and the next device fails, then the next. Finally the router itself becomes unavailable on MyDlink if I can't get to it fast enough, but with warning I can remote reboot and everything comes back online.
Unfortunately I cannot report what firmware is running on the router, the router is at a vacation home and I won't be able to visit the home for a week to go reset the router.
For what it is worth, have about 11-12 devices connected to the router via wifi
3 Dropcams (1 dropcam pro, two dropcam basic)
3 Dlink cameras (DCS-5009L, DCS 930, DCS 932)
1 Lenovo laptop
A Smartthings Hub
Liftmaster MyQ
2 Nest thermostats
1 Nest Protect
Synology server and an HP printer are ethernet connected.
It is not an interference problem or overheating or anything like that.
Next time I can get to the router I'll provide the firmware and I'll scan the logfile from the 880L which I have sent and recorded to the synology server and post if it provides any relevant data.
I'm going to replace the 880L until a fix is found, I can't afford to lose all my security cams for a week at a time when I'm away travelling.