Dear Thomas, we had similar issues on a x460G2, this is the config that finally worked for us;
configure vlan vlanName dhcp-address-range 10.10.10.2 - 10.10.10.254
configure vlan vlanName dhcp-lease-timer 3600
configure vlan vlanName dhcp-options default-gateway 10.10.10.1
configure vlan vlanName dhcp-options dns-server 8.8.8.8
enable dhcp ports portNumbers vlan vlanName
configure vlan vlanName dhcp-option code 15 string "internal.domain.com"
configure vlan vlanName dhcp-option code 119 string "internal.domain.com internal02.domain.com internal03.domain.com"
# alternative in HEX for code 15
configure vlan vlanName dhcp-option code 15 hex 69:6e:74:65:72:6e:61:6c:2e:64:6f:6d:61:69:6e:2e:63:6f:6d
And yes we did have code 15 in string and in hex before it finally worked
Hope this gives some guidance.
Regards,
Johann