@nkw1985 

That looks very much like Main Deco negotiating 100Mbps over Ethernet link to the ONT modem. When you reboot Main Deco, it'll briefly renegotiate gigabit speed but can't keep it due to noise/issues in wired link between it and modem.

How do you check speed uplink between Main Deco and ONT modem? 

Do you see "Abnormal WAN Port Speed" message in Deco app when speed drops?

Deco X50-POE has two Ethernet ports: 1× 2.5 Gbps port + 1× Gigabit port. Which one is used to connect to modem? If it is 2.5 Gbps port, I suggest to use Gigabit Ethernet port instead.

  @Alexandre.

Thanks for your reply.

Originally I was connected to the gigabit connection on the Deco and I did get the "abnormal WAN port error", so switched it the 2.5G connection, but has not made any difference.

I think its more than likely a noise issue as you mentioned. The DC power supply cable for the ONT was coiled up near the WAN outlet cable from the ONT to the Deco (I have now uncoiled this, in hope that this is the issue!) I may replace this WAN cable with screened cable if needed.

I monitor the internet speed from the Deco using a raspberry PI hardwired to the network then logging the data via Home Assistant. (so not actually seeing the ONT to the Deco speed)

  @nkw1985 

I would suggest switching Deco WAN link back to gigabit port. Not in this Deco model, but 2.5 GBps port caused some issues. Especially, if your modem has gigabit port, you would not want to introduce another uncertainty now or in the future.

"Abnormal WAN speed" message was introduced by TP-Link in Deco app for gigabit Ethernet ports not so long time ago. It were to specifically address the "why my Internet speed drops to 100Mbps but Deco app says all is OK." Consider this message an alert in Deco that its WAN link speed is now 100Mbps. There is no other way to see uplink speed in Deco app.

I am not sure if "Abnormal WAN speed" alert is enabled for 2.5GBps Ethernet port, and it is another reason to use X50-PoE gigabit port for WAN link to modem. At least until you fix the problem permanently. 

There is good news in all of that: if you see "Abnormal WAN speed" alert, the issue is definitely in Ethernet link between Deco and modem, and nowhere else.