Well the bad news is that most ROKU devices do not carry the settings to facilitate either VPN or smart DNS. The good news is that VPN enabled routers and repeaters are now cheaper than chips. A shameless sales pitch perhaps but we at VPN01 sell a VPN enabled router combined with a years unlimited data UK/US/CA/EU/IL VPN account for a very reasonable price. To see this, simply visit https://store.vpn01.com
The great thing about VPN routers / repeaters is that once configured, all devices (including your Roku) that are connected to that router / repeater will sit behind the country IP of your choice, supplying all devices your VPN account.