Roku sells a simple remote that works with all Roku TVs and the voice remotes work with Roku TVs as well. The simple remote is the cheapest option and it doesn't hurt to have a spare. You can use the mobile app as a remote as long as your TV is connected to your network, but it won't work to get a TV connected to a new network (if you move for instance or factory reset your TV).
Roku TV Remote | Replacement Remote For Roku TV | Roku
Roku Mobile App - Free for iOS® or Android™ | Roku
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