I am having problem connecting my tplink re450 extender's 5g band to my at&t router, can you help me with that?



First of all, you need to change the channel for 5G band on your at&t router and then try to connect the extender again. In order to change the channel on the at&t router, follow these steps:-
- connect your computer to the at&t router's network
- open your web browser and type this address:- 192.168.1.254
- It will take you to the web interface.
- Now click on the Home Network and then click on the WiFi option
- After that, it will ask for the device access code(that you will find on your at&t router itself)
- Enter the default access code and login to the settings
- then click on the Advanced option
- and from there, you can change the channel for the 5G band.