I guarantee I'm not the first one to be stung with this one 
I have the RXP GTN installed for P3D which is also based off of the Garmin trainer, and I was very pleased to see when I installed the TDS GTN that it picked up that install of the trainer so the install only took 30 seconds and I didn't end up with 2 trainers taking up drive space.
Nice, right up until I launched MSFS and up came an error about the GTN NXi trainer not being installed, please reinstall via the manager.
Several minutes of searching later I found that the manager is the installer, like most people in these days of decent broadband I just open installers without saving them so I didn't have it, easily solved, and it said the latest version of the Garmin trainer was definitely there.
Cue headscratching, then I had the idea to see if the NXi worked in the Garmin trainer and voila - the RXP unit needs the base Garmin GTN trainer and that is what the manager checks for, the TDS unit needs the GTN NXi add-on within the garmin trainer which isn't installed by default and the manager doesn't check for it. Once I sussed that out a single click on an 'install' button inside the trainer added that NXi trainer and we're over that hurdle.
Not the first, won't be the last, leaving this here for the next person that needs it
I have the RXP GTN installed for P3D which is also based off of the Garmin trainer, and I was very pleased to see when I installed the TDS GTN that it picked up that install of the trainer so the install only took 30 seconds and I didn't end up with 2 trainers taking up drive space.
Nice, right up until I launched MSFS and up came an error about the GTN NXi trainer not being installed, please reinstall via the manager.
Several minutes of searching later I found that the manager is the installer, like most people in these days of decent broadband I just open installers without saving them so I didn't have it, easily solved, and it said the latest version of the Garmin trainer was definitely there.
Cue headscratching, then I had the idea to see if the NXi worked in the Garmin trainer and voila - the RXP unit needs the base Garmin GTN trainer and that is what the manager checks for, the TDS unit needs the GTN NXi add-on within the garmin trainer which isn't installed by default and the manager doesn't check for it. Once I sussed that out a single click on an 'install' button inside the trainer added that NXi trainer and we're over that hurdle.
Not the first, won't be the last, leaving this here for the next person that needs it