@SanWin 

Deco is consumer grade WiFi mesh. What you see in Deco app is what you get. There is no channel selection option, for example.

I am assuming this is your first WiFi mesh, coming from standalone WiFi router. Older devices which do work just fine with standalone router might have issues connecting to WiFi mesh because they have problem with one of the following:

1. WiFi mesh: multiple WiFi nodes broadcasting same SSID/password;

2. Dual band: same SSID/password broadcasting on both 5GHz and 2.4GHz.

These would be their problem with WiFi mesh regardless of brand, not Deco mesh specific. 

Your Deco X55 has an option of running IoT network. Turn that on, leave with default 2.4GHz only band, see if smoke detectors can connect to it. See FAQ document: What is an IoT Network?

If still no luck, check WiFi Security settings (WPA, WPA2, WPA-PSK) on dlink router and set it same way on Deco IoT network.