Ralston18 :
Is the failure time consistent?
Are you sure that the IP lease time is not expiring via the main router?
After that I am going to suggest that the router is defective.
Heats up with use and then some component or connection inside fails.
Try using the router in a cooler location where it has plenty of air flow. See if the router stays operational for a longer period of time.
Also if the router has logs and the logs are enabled there may an error message therein that may help identify the problem.
Does the following link match your router?
https://www.manualslib.com/manual/1177385/D-Link-Dir-816.html#manual
If so, I did not note the availability of admin logs via the Table of Contents. Could be buried somewhere in the admin windows....
And the Troubleshooting section may be worth a look but your posted problem is unlikely to be addressed therein.
Is the failure time consistent?
no, some times after i switch off the dir 816 router for a minute or less
some times after i switch off the router for second time
some times after a day
i'm sure the problem isn't because hot or cold
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Are you sure that the IP lease time is not expiring via the main router?
yes, because i used the main router with d-link dap-1360 and worked perfectly all time
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Does the following link match your router?
yes exactly, and my router has not logs