Deco's Bluetooth is for the initial setup process only, as soon as the setup finishes, the Deco will turn off its Bluetooth, I had been scratching my head over this for hours before I got this answer from TP-Link.
If you are having trouble to connect Bluetooth during setup, here are my suggestions:
1, Make sure your mobile phones are not in power saving mode.
2, when setting up one Deco, keep the rest of the Deco off except for the one your are setting up and the main Deco.
3, keep your mobile close to the Deco you are setting up.
4, try it with another Deco in the set.

Good luck.

Thanks for that. Already tried all suggestions before starting this thread. I gave up and took the units back for a refund.

Eight months later... I'm in the same boat. Has TP-Link fixed the problem yet? If not, I'll just return the product.

Why no direct web-based interfaces? They work...

Merci!