It seems that most TVs these days only have one button. It often serves multiple purposes. Once you turn the TV on, tap the button and you may get a menu of things you can do with that one button, but volume control may not be one of them. You should be able to use the Roku mobile app as a remote and it will be able to adjust the volume (as long as your TV is connected to the same network as your phone/tablet).
Roku Mobile App - Free for iOS® or Android™ | Roku
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