Powerline compatibiliuty

@AlistairL

Welcome to Zyxel forum and sharing your user experience.
Those two models are compatible to each other, and the connection is allowed to be established between them. However there are some points you need to take care of when mounting our PLA products:

1. Are these models are deployed in the same electric circuit? Since PLA deliver signals through power line, it requires both models located in the same circuit. You can do the test for pairing 5405v2 and 5206 in the same extension cord, if they still can't pairing each other, then it could be the hardware issue.

2. Did you trigger the pairing process successfully? To pair PLA models, please first press the "Rest/Encrypt" Button on the PLA which is connecting to the home router until its power LED starts blinking, then repeat the same step on the other PLA located in the other place. Once both PLA models' power LED are blinking, they'll trying to negotiate and pair each other, then you'll see the second LED(Homeplug LED) is blinking, with different color stands for the network speed.

3. Check the LED status to verify your PLA's current status.
Directly observe PLA's LED behavior is a good way to know its status. You can check the LED meaning in our User Guide p.7~p.10. Also it's welcome to tell me what's your PLA's condition here, and then I'll be glad to help you.
(User Guid's linke: https://download.zyxel.com/PLA5456/user_guide/PLA5456_v7.0.2_Ed6.pdf)

*Noted:  When there's no connected devices in PLA's LAN port, the model will then turn to  standby mode for power saving purpose. Which is also a common reason for failure pairing. (Your can find more description in User Guide page.9)

Best Regards,
Richard