@Kozicki 

Hi, welcome to the community.

Have you checked the internet connection type on the Deco APP>internet connection here? Is it Dynamic IP?

The other possible reason would be the service provider has bound a specific MAC address to the internet service.

By the way, how many Ethernet ports do you have on the modem? Can I have the model number of your ISP modem?

I used to see users mention that if there is more than one Ethernet port on the modem, you could only use one by default. If you need to connect more than one device to the modem you might need to contact the provider to activate other ports.

@luotinen 

Hi, I think your issue is not the same as Kozicki. 

For your case, it is suggested to change a different Deco as the main Deco in case the first one is broken for some reason:

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

Thank you very much.

Best regards.