I am not sure if elevation is important for TrainerRoad training, but I wanted to report this problem. Garmin 830 apparently has a design flaw in barometric altimeter. Many people have reported this on Garmin forum. For me it was working fine until something broke in January. I think this was a result of me riding in the rain. I was trying to blow the air into the altimeter, as people suggest but this did not help. Garmin records only half of elevation gain. On Garmin connect I can fix this using elevation correction, but on TrainerRoad it still shows the original data. Again this is probably not an issue since I am still using the power meter, but is there a fix for this?
My 800, 1000 and 1030 worked out of the box but my 130+ is giving me some funny data. It might be the same issue you are having. Apparently, I have to calibrate it. It doesn’t matter for TR indoor rides as there’s no elevation but it just looks weird for outdoors TR, races and leisure rides.
My Garmin 830 also worked out of the box for a few years until it broke. And Yes, I tried to set the elevation in altimeter settings multiple times. The thing is broken. Actually Garmin knows about the problem as explained in this video.
That’s annoying. I take it your unit is out of warranty! They once replaced my old 305 that failed out of warranty as it was “a common problem” :-/