Enter the primary and secondary DNS server addresses on their own separate line. Once you’ve typed the IP addresses, be sure to either press Tab or click outside the text field after typing the second address. If you close the window while typing the second IP address, it saves only the first IP address and not the second one.
https://www.howtogeek.com/204672/how-to-change-the-dns-server-on-a-chromebook/

