Really slow network speed when ex2 Ultra is connected to Router

If got a problem and i hope to find a solution here.
When I connect my ex2 ultra via network cable and thunderbolt adaptor to my macbook pro (mid2014) everything runs finde. i get a transferspeed of about 50 mb/s what works fine for me.
If i plug the network cable to my router, and another cable from the router to my macbook i still get a connection, but i only achieve really slow network speed. Highest was 2mb/s.
Router is a Fritzbox 7360.
Yes, both LAN Ports are in Gigabit mode
Yes all used cables are cat5e or better.
I even tried a network switch (hp procurve 2324), but when using it i get no connection to the NAS.
When connected to the router, i can easily connect the NAS via web interface and it seems to be much faster than when i connect from the mac.

Has anyone an Idea what my problems solution could be?
I must admit, that my skills in networks tend to beminus 8 on a 1 to 10 scale…

Hi JanWho,

You can refer below mentioned article in this concern: