Sudden decrease in download speeds

Hi - I bought an Orbi system about 6 weeks ago and it worked well until last week when I noticed that the download speed has dropped from around 56Mb/s to around 20Mb/s.  When I begin the speed test the download immediately peaks at 56Mb/s and then progressively drops to a minimum.  Upload speed remain at around 23Mb/s.

I have re-installed the latest software, did a new setup and tried both router and AP options without any change.  When I connect to my modem/router i get full speeds up and down with no problems.

Can anyone help me with this?