@Jman66 

Hello, have you checked this article to confirm all the basic settings?

https://www.tp-link.com/support/faq/2809/

If you have checked all the settings as we suggested in the above article but the Tether app still cannot find the router, please tell us the below information, we will confirm:

1. Hardware version and firmware version of the Archer C5400X. You can find the hardware version on the router bottom label, and the firmware version can be found on the router web GUI:

https://www.tp-link.com/support/faq/315/

2. OS version of the phone that runs the Tether app.

3. MAC address of the C5400X (you can provide this via PM).