@Mano11 

If you select dynamic IP as the internet connection type, then you should be able to get internet from the streamyx modem with wired cable connected directly. 

First, please connect your computer to the streamyx modem directly and verify whether you can get internet and click here to get the IP address and gateway. 

A. If you can get internet, then connect the computer back to the Deco M5 by wired cable and reconfigure it, this time, please clone the computer's MAC address. 

B. If not, the internet connection type will not be dynamic IP, instead, it can be PPPoE or something else, try to contact the ISP to confirm it and get the configuration parameters needed. 

Second, try to swap the locations of the Deco units, like changing the slave Deco to the main Deco to give it a go. 

https://www.tp-link.com/support/faq/2400/

Any updates, please keep me posted.