What is very strange here is that your Estimated speed range for VDSL (FTTC) at the top of the checker are very good, 67-80 on a good line, unaffected by crosstalk and even on a poor line with a lot of Crosstalk you should get 60-80Mbps. Yet your provider estimated your speed as very low, with the estimate of 4.9-15.3Mbps. It's all very strange. I suspect you either have a fair bit of Aluminium on your line somewhere or you're not connected to the cabinet Openreach and BT think you are. On the checker it will tell you the cabinet you're supposed to be connected to at the top. I wouldn't be surprised to find out you're connected to a cabinet further away and there's one closer, likely within 2-300 yards.