If you want more reliable signal, I'd invest in your own hotspot. CGs are getting better with their WiFi, but they are still iffy, and you may have one with a very weak signal, and the Roku would probably not work well there.
Personally, I'd get the Hotspot (which usually has a trial period for a week or so), and take it out for a trip...try it, and then decide if you'd do better with DVDs, or downloading movies to your computer, if the signal isn't enough for the Roku. At least that way, unless you have other uses for the hotspot (I take mine to work on the tugboat), you won't get stuck with a monthly fee and contract.
Good luck:)