@Moby58
What happens is Satellite Deco can't keep wired backhaul connection, so it does switch to WiFi backhaul. When you reboot the system Satellite Deco can briefly negotiate Ethernet backhaul link, but due to some issues with it switches to WiFi backhaul. The issues could be problematic cable by itself or electromagnetic noise near it.
With TP-Link Deco it is hard to troubleshoot such issues. Even if you check logs through WebUI, the most they'll tell you is when Satellite Deco switched to WiFi backhaul, but not why.
Except obvious "replace the cable" which may help or not, I can only suggest one other thing. Get gigabit networking switch from TP-Link brand that does have LED for connection it establishes, place it in the house and connect to one end of that cable. Example of such switch:
If will be [Main Deco]<--Ethernet-->[TL-SG105E]<--40m Ethernet cable-->[Satellite Deco]
Two thing you'll get from this: you'll see negotiated speed of wired link and also perhaps the switch will help to make connection stable.
If that alone does not help, get second switch of same model and place it in the shed, connected to the second end of Ethernet cable. If switches can keep the link, you have your workaround for about $50 USD at current price of pair of these switches in my region on Amazon. If they can't keep wired link, you'll see it by their LEDs and it will be no longer Deco mesh problem.