Mijia Bedside Lamp gen 2 loses connection after restarting app

Hi,

I connected my Mijia Bedside lamp with the Yeelight app (on my Xiaomi Mi6), everything worked perfectly. However after I close the app, I can’t connect to the lamp anymore. In the app the lamp shows as offline (however it is working and I still control it with the finger gestures).
When I reset the lamp, I can once again add the lamp to the app and after reinstalling it, I can use it again. However, when I close the app again, the same thing happens.
When I check my router, the Yeelight is listed as a connected device.
When I ping the Yeelight from my computer, this is the result:

Pinging 192.168.0.156 with 32 bytes of data:
Reply from 192.168.0.170: Destination host unreachable.
Reply from 192.168.0.170: Destination host unreachable.
Reply from 192.168.0.170: Destination host unreachable.
Reply from 192.168.0.170: Destination host unreachable.

Ping statistics for 192.168.0.156:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

The same thing happens with the Mi Home app, and when I try this with the OnePlus 5 of my girlfriend.
I have no other Xiaomi smart LED bulbs/lamps.
Does anyone know what the problem is and how I can fix it?
Thanks!

Have you tried with your 4G network?