Raspi losing frequently connection to internet (Fritzbox 7490)

Hardware Server
1

Hi all,

my raspberry is losing frequently to internet. All other clients are working fine only raspi is affected. I believe it has to die with DNS or my Fritzbox 7490. When I restart Fritzbox it usually works again. Has anybody a solution for that?

Thanks.

 sudo apt-get update
Err:1 https://apt.homegear.eu/Debian stretch/ InRelease                        
Could not resolve host: apt.homegear.eu
Err:2 https://dl.bintray.com/openhab/apt-repo2 stable InRelease                
Could not resolve host: dl.bintray.com