dhclient is not getting an lease / ip address from dhcp - but windows gets one

How could it be that I get easily an ip address with Windows 10 network manager and not with simple Linux QNAP NAS System? And not working situation. I tried to renew the ip address manually.

[admin@NAS ~]# dhclient -v eth3
Internet Systems Consortium DHCP Client 4.3.4
Copyright 2004-2016 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/ 

Listening on LPF/eth3/24:5e:be:35:c3:23
Sending on   LPF/eth3/24:5e:be:35:c3:23
Sending on   Socket/fallback
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 9
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 11
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 18
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 13
DHCPDISCOVER on eth3 to 255.255.255.255 port 67 interval 3
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

$ ethtool eth3

Settings for eth3:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: Symmetric
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  1000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        MDI-X: on (auto)
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes

I tried to set the network adapter to static with the IP address from Windows. Then I tried ping via ping -I eth3 8.8.8.8 which didn't worked, too. I tried different ports, too - which results in different MAC addresses. I dont know if the QNAP is to stupid to see the router or maybe the router ignores the QNAP. Or Linux dhclient is much different than Windows DHCP so that on windows it works and on linux not. Maybe someone of you have few ideas :) Thanks in Advance!