Multy U. Problems with Netatmo Weather Station falling in&out

Which netatmo device you have?
Do the other host also disconnect when your netatmo disconnects?

Here's the advanced troubleshooting guide for netamo
https://helpcenter.netatmo.com/en-us/smart-home-weather-station-and-accessories/connection-wi-fi-radio/advanced-network-troubleshooting-guide

In troubleshooting guide it says ideal distance is 1 - 2m.

Then put netatmo closer to multy (make sure radio signal doesn't block by obstacles) and run inSSIDer check if multy's radio signal gets weaker or change channels with inSSIDer.

That way you can have more clear idea about what's going on.