@Domly Hi,

As some of your devices will not accept router password, please contact Router support first and  your Wi-Fi password or change router's Wi-Fi password back to the original one.

If you have a PC wired to power line extender, you could use tpPLC Utility to change Wireless settings as below:

If you have to connect  by Wireless,you could reset one of the Powerline extender by press reset hole for 8 secs while it is plugged into the wall , and add it back to your powerline network.  Later use tpPLC Utility/ APP to change the wireless password:

https://www.tp-link.com/support/faq/1576/

https://www.tp-link.com/support/faq/1611/

Sometimes when uses pressed the WPS button to synchronize the SSID and password , there is small chance that router's password change to a random password.

Here is a related FAQ to explain: Why is my main router’s wireless password changed after using WPS to set up the Range Extender?

https://www.tp-link.com/support/faq/765/