@Alex_Led 

Hi,

What is the status of the "Internet" light on the AX1500 router when it is plugged into that Ethernet cable?   (i.e. off, orange or green)

  @woozle it isn't on at all 

  @Alex_Led 

Ok, that would mean there is no physical connection.

Please verify that the Ethernet cable that is going from the living room to the outside office is securely plugged into one of the LAN Ethernet ports of the YouFibre router. The LAN Ethernet ports are usually marked with "LAN" and are often made from yellow material.

Are you able to follow the pathway of that Ethernet cable all the way from the YouFibre router to the Archer AX1500 router?

Please do your best to verify that the Ethernet cable that you think is the one going from the living room to the outside office is indeed the correct cable.

  @woozle thanks, I think that's the problem, I cannot find any cable that goes from the garden into the house! But the previous neighbour said he just plugged in the garden router into the Ethernet outlet in the office..I wonder if his previous network provider did something there..

  @Alex_Led 

Can you find an Ethernet outlet inside the house that is similar to the one in the office?

If the previous owner had his Internet router inside the house and a working Internet connection at the outside office, then there must be a cable between the house and the outside office. Unless the previous owner had the network provider install two separate Internet connections (and thus pay twice), which seems unlikely.