ReadyNAS 102 won't let me set DNS servers

I guess that you disabled DHCP in your network,to manually set DNS server please enable root SSH access and follow the steps as below

e.g, change DNS to 8.8.8.8:
root@nas-26-D7-28:~# connmanctl
connmanctl> services (check eth)
*AO Wired ethernet_841b5e26d728_cable
*AR Wired ethernet_00ac5ac7374b_cable
connmanctl> services ethernet_841b5e26d728_cable (check settings)
/net/connman/service/ethernet_841b5e26d728_cable
Type = ethernet
Security = [ ]
State = online
Favorite = True
Immutable = False
AutoConnect = True
Name = Wired
Ethernet = [ Method=auto, Interface=eth0, Address=84:1B:5E:26:D7:28, MTU=1500 ]
IPv4 = [ Method=dhcp, Address=10.40.1.78, Netmask=255.255.254.0, Gateway=10.40.0.13 ]
IPv4.Configuration = [ Method=dhcp ]
IPv6 = [ ]
IPv6.Configuration = [ Method=off ]
Nameservers = [ 10.40.0.6 ] (we need to change to 8.8.8.8)
Nameservers.Configuration = [ 10.40.0.6 ]

connmanctl> config ethernet_841b5e26d728_cable nameservers 8.8.8.8
connmanctl> services ethernet_841b5e26d728_cable
……
IPv6.Configuration = [ Method=off ]
Nameservers = [ 8.8.8.8 ] (it’s done)
Nameservers.Configuration = [ 8.8.8.8 ]