I had to disable Fast Roaming on my Deco after struggling A LOT trying to understand why the heck my HP DeskJet 2774 refused to connect at its the last setup step, given it already knew the old SSID and passwords. 

Through another topic[1] I found the issue was Fast Roaming. Turning it off automatically made the printer connect 🤡 

But then, disabling Mesh for it didn't made a difference: reenabling Fast Roaming disconnected the printer. 

What's the difference between those two options? How they interact? 

You can be as technical as possible in the answer. I saw something about 802.11k/v/r but didn't get much since the TP-Link blog forced a bad translation on my device lol

[1]: https://community.tp-link.com/en/home/forum/topic/237370