Smart LED S1 Cannot connect to Alexa

I got pair of model#YLDP13YL color light bulbs, and there is no problem to connect those bulbs to Homkit through Yeelight app in iOS. However, I tried to connect them to Alexa by using the Yeelight skill, there is no device cannot be discovered. I tried to connect those bulbs to different server in Yeelight app, but there is still no way that I can connect them to Alexa. If there is any solution to solve it? Thanks.