Thank you for posting in Wired Communities. Please refer to the information below:
1) If you change the speed of I219-V to 1 Gb, please ensure the Netgear router where your I219-V connects is running at 1 Gb mode as well. Please ensure the NIC and the router should have the same speed setting.
2) You may also try configure the advance setting on the NIC, please refer to this link for reference:
3) Please try changing the cable to test in case you have spare cable.
4) As this is an onboard NIC on Asus board, it is recommended to obtain the customized driver for the onboard NIC since OEM board vendor might have made changes on the NIC to adapt to their system. Here is the link for you to download the driver:
https://www.asus.com/Motherboards/Z170-PRO-GAMING/HelpDesk_Download/ (Please note this is third party site which Intel has no control over the content therein, please contact Asus* if you have any inquiries about the website content)
Below is the link provided for your reference about our support option for Ethernet controller
Hope the above information help.
You may try visit below Netgear forum and try out the suggestion given and post on that site:
You can also try connecting the I219-V to another system via back-to-back connection to test the connectivity to check if the speed can sustain at 1Gbps.
Feel free to update me.