@StijnDeco 

Hello Stijn, what is the current network diagram? How do you connect these devices together? 

Cause the TV works well when Deco works in AP mode, I'm afraid that we need open the port for it to make it access internet.

Have you ever done any settings on the main router  to make the TV receiver work? Like port forwarding.

With the 2nd one connected to the Deco, what is the IP address and gateway obtained by it? 

In the meantime, it is recommended to contact the tech support of TV receiver to get some specific help cause they are more professional about their own products. 

Good day. 

@StijnDeco 

Thanks for your reply.

1. Try to disable the antivirus feature via the Deco app;

2. Please verify the IP address and gateway of the TV receiver.

3. Any special settings need to do to make it work per TV's tech supports' advice? Like port forwarding? Below is the instruction to open port on the Deco. 

https://www.tp-link.com/support/faq/1797/

Note: While cause there is an initial router existed, you need open the port for the Deco at the same time to create a tunnel for the receiver if needed. 

4. Besides, have you ever tried to configure the initial router as bridge mode and Deco M5 as main router to avoid the double NAT issue? 

Good day. 

@StijnDeco 

Hello Stijn, just take your time to test it.

Any updates, please let us know.

Best regards.