Per above, this is limited due to the single-speed setup and lack of virtual shifting in TR.
ERG is fully functional.
Resistance and/or Standard could also be used, but will require manual adjustment of the setting within the app range coupled with cadence changes. Definitely a hack but sort of parallels virtual shifting to a degree.
The more compatible option to TR is the Elite Square, as the virtual shifting that Elite implemented will work in resistance / sim mode for any app, and not just Zwift like with the Zwift Ride