Inconsistent download speeds?

I upgraded from standard to broadband to fibre optic broadband about 6 months ago. To begin with we were getting about 10 Mbps upload speed and 40 Mbps download speed. However, for the last couple of months I've only been getting around 10 Mbps download speed but the same upload speed as before (10 mbps).

I've used the diagnostic function in my router and it passes all the tests except for 'ping default gateway'. Not sure if this has anything to do with the problems though.

My router is a TP-Link TD-W9980 and our ISP is plusnet.

Any suggestions on how to fix the problem would be greatly appreciated.

Thank you!