@LornaBee 

Hi, since AC200 is not the model number, have you seen any models like Deco M4 or S4?

If the LED was red blinking, this unit should be offline from the others?

How about the LED lights on other Deco units? Are they solid white?

And if this Mesh system was configured by your son before, it is suggested to check the email account on the Deco APP and you might need to make sure you have signed into the original email which was initially used to set up Deco.

Thank you very much and best regards.