Suddenly no connection of some of my bulbs

I do have 7 bulbs. Yesterday one of them did not have connection anymore. Today 2 other bulbs are disconnected too.
I tried to reconnect, no succes. I did a reset, no succes. It is not possible to connect to the router by the Yeelight app. If i try to connect those bulbs by the Xiaomi Home app, the do connect to the router.
What is wrong with the Yeelight app?