Sudden Speed and Reliability problems

Hi 

I am on the 2100/300 plan, but have had constant speed and reliability issues with our internet lately.

Doing a speed test its gets as low as 88.5mps download and on average 500mbps compared to being able to get 2.24gbps when I did a speed test in March.

At times upload speed significantly faster :) 

I have tried restarting and checking all wiring and no change. 

My setup is the xb8 bridged to a deco tp-link wifi. It always has been this setup for the longest time so no idea why the sudden degradation in service.

Help and advice appreciated.

My router keeps track of speed checks, see attached screenshots. As you can see the last time I got the correct speed was March!

for results.