Found a link to (hopefully) sign up for streaming Davis Cup. It's like digging for gold finding this thing. Why it's not easily found on the Davis Cup site we'll never know. I don't think there's an app, so it's likely in browser or casting it from your device to your TV, assuming the legend appears when you play it. I haven't signed up for it yet, not sure that I will. Reports on Reddit are that it is glitchy and/or bad.

https://sports.rakuten.com/daviscup-landing-en


And FYI, Fox Sports App has nothing in the way of Davis Cup up in their app today unfortunately.