It's a known issue with the Unity input manager and some versions of OpenTX.
Yes you are right, you need to setup the endpoints on your radio to go from 0 to +100 (instead of - 100 to +100)
You can find information on how to get your controller set up in the Taranis setup guide.
(There are also a lot of youtube videos that shows how to do it).
You can also find some general tips in the user manual.