@Anonymous User
Welcome to the forum. You should be able to set up your iPhone as a personal wifi hotspot by following these steps
https://support.apple.com/en-gb/HT204023
which your Roku can then connect to. However you need to be aware that NOW TV use adaptive bitrate management when streaming, and the Roku will try to draw as much data as your connection allows to get the best picture possible, which will result in anything up to almost a couple of GB of data each hour if you have a good 3G/4G connection on your phone. Unless you have unlimited mobile data you're probably going to find this uses up your data allowance very quickly and could end up costing you a lot in data charges.
Unfortunately NOW TV doesn't have any options to reduce data usage or select 'mobile' data streams so other people in your situation have found tethering to a phone for NOW TV not to be a viable option.
