What lights are lit on the ONT ?, the Power and PON lights should be solid green, if the red LOS ( loss of signal ) light is lit or the PON light flashing green then there is an issue.
If there is already a pre provided Ethernet cable in the ONT , simply connect the other end to the appropriate router port.
If the PON is steady green, then connecting an Ethernet cable between the ONT and router ( WAN ) port is all that is necessary at the customer end, if it doesn’t work , then either your ISP or Openreach need to do something remotely, not at your property
