@Pieraerox  Hello, 

Thank you for your detailed information, you could try suggestions here and see if it helps with the issue: 

1. Please factory reset your powerline unit by tpPLC Utility one by one:
https://www.tp-link.com/support/faq/1612/  

2. Put 2 powerline unit in the wall sockets, in the same room, and pair them again :

Paring Note:  press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again)
When the Powerline Led or House Leds are on, the pairing process is done

How about the Leds status on the main powerline unit and the extender unit? Do you mean the House Led would not light up, or they are able to pair but lost connection again? 

The warranty policy could be different in different areas, you could try to contact local support here to check  warranty details: 

https://www.tp-link.com/support/contact-technical-support/#Hotline-Support

@Pieraerox  Thank you for your update, 

Please locate the powerline units in the same room in the wall socket ( or plug powerline devices in a same power strip ) and see if powerline device lost Internet after 5 mins. 

Is there any new device being added in your network that may cause this issue?  If not,  from your description, sorry to tell you it's more like a defective the power line unit (s), it is recommend to  contact local support or store to check warranty .