@Ultramarinus 

Hi, for your issue, please check the below information.

You want to connect RE450 and powerline device by Ethernet cable, then RE450 can provide the stable internet connection for your clients, right?

Theoretically, this can be achieved. You need to change the operation mode of the RE450 to be Access Point.

Also, you need to make sure if the powerline devices work well.

If the powerline units are working well, you can follow the instruction to set up the RE450 to be Access Point. https://www.tp-link.com/support/faq/1143/

Note: The performance of RE450 will be influenced by the powerline units if the RE450 is connected to the PLC unit by cable.

If the two devices can't work together, please confirm the below information for us:

1. the model number of the powerline unit. 

2. How do you connect the powerline units in your house and how about the lights' status on each powerline unit and RE450.

Look forward to your reply.

Good day!