The modem has a security mechanism.
If the router keeps working for a period of time, the modem will block clients to access the config page.
Only a restart can let a client to login again.

mohamadreza wrote

Model :

Hardware Version :

Firmware Version :

ISP :

hi
i have a modem( TD-W8151N).
this problem is permanent that i can't open config page(192.168.1.1)........
i have to reset the modem.and few days later this problem will repeat again

what should i do?

By "reset" you mean turning it off and on, or you mean factory resetting it with a pin?

If it's the second one, you could try updating the firmware.

Reset the modem, check the firmware's version, and then check TP-Link's website. If it's not the latest version, update it. This might solve the problem.

CAUTION: Make sure you download the correct version. There are different hardware reversions of your modem, V3, V4, etc. Be absolutely sure you get the correct one.

P.S. I also have this issue.