@Sam_Maran 

Hi, thanks for posting question here.

May I confirm how log you have used this TP-Link DSL modem router? And how many wireless clients do you have?

Is it all wireless clients disconnect from network, or only certain client acts such performance?

Usually. if wired connection is stable, only wireless connection drops, you may try to change the wireless channel and channel width:2.4GHz→20MHz+Channel 1/6/11;5GHz→40Mhz+channel 36/48.

Also, Make sure the firmware is up-to-date.

  @Marvin_S 

It's on auto now. This router has only 2.4Ghz .

Should I leave channel and channel width on auto? Or would it be better to change?

  @Sam_Maran 

Hi, you may change as suggested before. Aslo, make sure firmware is the latest.

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If you can find the user manual or product page for your TD-W9960 router, it might specify the maximum recommended number of connected devices.