My teenagers are swimmers and biathletes, but they could use my new trainer once a week for some cardio training.
I can’t spend 40$/month for them just to use my new wahoo trainer?
Since I bought it with a swift cog it can only be used with swift I get it correctly? Is my only option to by a cassette to the trainer and skip the swift cog or can I use the swift cog in another way? (Another app or with swift basic somehow).
You have multiple options depending on what the kids want to do.
Easy - just take the cog off and put a cassette on if you’re using a bike on the trainer and not a Zwift Ride. Shifting is the same as on the road.
Even easier - Wahoo app can run a workout in ERG mode. Though you can do this with a cassette on as well. Note that having the cassette on does nothing to make your virtual shifting experience any worse.
You can try other apps that have free trials. KinoMap is a good one for riding real-world routes, for instance.