@YLun 

Hello, thanks for reporting the issue to the community.

To better assist you, please provide us with the below information:

1. What is the operating system of the computer? Please refer to this link to check the wireless link speed on the TX50E when it is connected to the router's 5GHz network if it is a Windows computer:

https://www.tp-link.com/support/faq/2265/

2. Are you testing this 5GHz speed on your smartphone in the same location of this computer? Try to adjust the antenna on the TX50E to see if the link speed will be better.

3. It is suggested to try to change the wireless channel and channel width on the wireless router, which may help to improve the wireless connectivity and range. BTW, what is the model number of the AC1200?

If none of the above helps, please refer to the below article to review all the settings in your network and get back to us with the required info, then we will try to investigate further:

https://www.tp-link.com/support/faq/709/

Good day.