@Summer_Solstice
In Deco app, if you open info page for Satellite X20 in the attic, it'll tell you the quality of link between it and rest of Deco mesh. Example:
If you see Weak or Poor Signal, that could mean Satellite Deco is too far and can't establish good connection to the rest of Deco mesh. That'll prove your suspicion. If Signal is Fair or Strong, the issue with WiFi cutting off might be somewhere else.
This page will also show you where Satellite X20 is connected to. Often, Satellite Deco prefers connection to Main Deco, even if there is another Satellite Deco closer to it. If attic X20 connects to Main Deco which is few floors below, that could case issues with WiFi stability.
For some Deco models with proper firmware that connection can be configured manually. See the following document: How to select a preferred signal source on the Deco App
If you have that feature in Deco app, use it to connect attic X20 to Satellite Deco which is the closest to it.
Other than that, using powerline link is a good idea. You don't need to buy P9s for that. Just get pair of TP-Link powerline adapters, place one in attic and one near Main Deco. If these powerline adapters can establish link with each other, connect attic X20 to attic powerline adapter and second Ethernet port of Main Deco to powerline adapter near it. If everything works like it should, you'll see in Deco app that attic X20 is now connected to Main Deco through Ethernet.
I would recommend TP-Link Powerline Ethernet Adapter Starter Kit - AV1000 (TL-PA7017 KIT).