Yesterday I had talktalk Fibre to the premises installed with a 100 Mbps connection.

I was pleasantly surprised to see that this connection seems to include a little modem on the wall and the TalkTalk provided router connects to it via its WAN port (as set up by the install engineer)

I have a 3 x Deco M9 Plus mesh wifi system and I tried unplugging and removing the TalkTalk router and directly connecting the ethernet out of the main M9 into the modem on the wall and it worked first time! I did a Speedtest.net and got 109 Mbps on my phone - all good so far!

Later last night things seemed to slow down a bit and I did another Speedtest.net from the same position - this time I only got 40Mbps on my phone and it has been like that ever since

I did a Speed test using the Deco app (which is testing the speed between the modem and the router I presume) and it is still showing as around 100 Mbps - but I can no longer get more than 40 Mbps or so for any devices on my network.

Can anyone suggest what the reason for this might be and what steps I might take to fix it please? Seems odd that speed between modem and router is 100 but speed between any device and router is only 40 - particularly when I intially got 109Mbps.
 

thanks.