My ISP speed is 200MBs - gets 200MBs from samsung galaxy S8 througth WIFI.

When I connect CAT6 etherent cable to a computer (tried 2 computers with Ubuntu Linux and Windows) the speed negotiated down to 100MBs:

$>sudo mii-tool -v eth0

eth0: negotiated 100baseTx-FD, link ok

product info: vendor 00:07:32, model 17 rev 2  

basic mode:   autonegotiation enabled  

basic status: autonegotiation complete, link ok  

capabilities: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD  

advertising:  1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control  

link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD

$>lspci | grep Eth

03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 03)

$>sudo ethtool eth0
Settings for eth0:
   Supported ports: [ TP ]
   Supported link modes:   10baseT/Half 10baseT/Full 
                           100baseT/Half 100baseT/Full 
                           1000baseT/Full 
   Supported pause frame use: No
   Supports auto-negotiation: Yes
   Supported FEC modes: Not reported
   Advertised link modes:  10baseT/Half 10baseT/Full 
                           100baseT/Half 100baseT/Full 
                           1000baseT/Full 
   Advertised pause frame use: No
   Advertised auto-negotiation: Yes
   Advertised FEC modes: Not reported
   Speed: 100Mb/s
   Duplex: Full
   Port: Twisted Pair
   PHYAD: 0
   Transceiver: internal
   Auto-negotiation: on
   MDI-X: Unknown
   Supports Wake-on: pumbg
   Wake-on: g
   Current message level: 0x00000033 (51)
                drv probe ifdown ifup
   Link detected: yes

There is no problem with the computer\cable - because when connecting both computers with the same cable the speed is 1G.

So there is no problem with the computers etherent and the cable. Tried to change port in the router and validated QoS is turned off.

Do you have more ideas, why is this happening?