New tv

DirecTV is designed to have a box (receiver/HDDVR or Client). Depends on what models your current boxes are on what exact options you are permitted (as equipment options changed in the last few years per AT&T, which hopefully will change to be more flexible now that AT&T is leaving the TV business). As normal, this would add $7 to your monthly bill for one more TV total.

With a dedicated streaming box (Roku, Firestick, etc.), and decent internet, you can use the DirecTV app, but it will be a limited experience as it is still satellite primarily (internet is secondary perks). It would not have all the channels and content because of this.

If the spare bedroom is not used often, then just using the app may be enough. But if you have guests a lot, or just those with tastes the app cannot cover, then going with the normal with a box would be the better option.