@CharlieOz 

Hello，

1. Please change different devices connect to the 5G network, then check if all the devices are having this issue.

If all the devices all having this issue, may I know the model and operating system of them?

And please send me a screenshot of the error information, thank you!

2. Please do not set the password for 5G, just leave it open,  are you able to use it?

3. Try to set up a simple password, like 11111111, did it still have this issue?

4. When did you purchase this product and when did this issue happen? Is it working well before?

5. Please try to change a different channel of the router.

How to change:https://www.tp-link.com/en/support/faq/2794/

6. Check the hardware version and firmware version of the router.

Hardware: Please find the hardware version at the bottom label of the product, which begins with Ver.xx.

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

May it help and have a nice day!