[Problem] Latency and lags by 3490 VDSL2 connection

Hi all,

I have one problem with my new modem 3490 :(

All tests I did with disabled firewall "Exposed Hosts" and same fritzbox settings, have only switched over modem and router mode.
These problems only in modem mode 3490, when I use old modem and fritzbox as router over "Existing connection over LAN" is everything works fine.

ISP: Swisscom 20000/2000kbit
New Modem: 3490 A/CH firmware 6.20 (connected as VDSL2 annex a)
Old Modem: Motorola 7340-44-20SC

Problem 1: In online gaming connection not stable with big latency and many disconnections, it's unplayable.
Problem 2: Not working teamspeak it connected and every 3-5 sec. disconnected and so in the loop.

Official AVM support can not help, they only said: very sorry to hear this...

Please help to understand what exactly the problem and how to fix it!

[Problem] Latency and lags by 3490 VDSL2 connection