Have you tried to turn off "fast roaming". Try turning that off, and see if your device roams to it successfully. Mine had issues with 2 X50 and 1 X20, when fast roaming turned on, it will have...

are you using WPA3? or WPA2? In certain cases, one of my friend had devices slow down or total entire disconnection in the house and most devices will fail to even connect despite the router is...

Use the signal source and connection preference to adjust for the devices part

yes, use the deco app to add in deco x10 or E4

Yes please

Perhaps that is the maximum it can connect to reach contrary to the website details about it reaching up to 100 devices. Consider upgrading the device then