T58 A Sip Phone looses connection

[Pepejans]

Hi everyone,

I use a SIP Yealink T58A together with a brand new Fritz Box 7590 AX. Everything is connected and very often after a short period the phone looses the IP Connection to the Fritz Box. Then it shows "no service" and tries to reconnect. Sometimes it works but mainly not and like this the phone is for me usesless. Only after a couple of minutes, when the phone starts, it connects first and then looses the ip connection again. I installed it after several studies over youtube and don't find any mistake. This is the only IP Phone I'm using so not in a company surrounding with a network. Might anyone have an idea, where I need to adjust anything differently? Internet and an old IP phone, what I used for many years before, never had those problems. Thanks a lot in advance