Which siren are you using? If it is the utilitech I don’t think you can flash that light independent of the siren. Maybe take siren off your flash list.
Also I never had good response using that Smart Security App. I would suggest trying “Smart Alarm” in the shared apps.
I have an Fortrezz SSA2USR Siren Strobe Alarm Wireless Z-wave Red Lense
Sold by: The Smartest House
I using the Smart Security from “More/Safty & Security” is that the one you said?