@Sa90
Hi, for your issue, we would like to confirm the below information first.
May I know which mode is used for the device TL-WA901ND?
How do you want to connect this device and your main router?
Can you explain the issue more clearly for us? May I know what troubleshooting steps you have done to make it work?
BTW, may I know when and where you bought the device?
Look forward to your reply.
Have a nice day!