The app is has two offerings; a free version with limited use of YouTube' API data (meaning you get less keywords searches etc) and the Unlimited version which costs about $3 per month. That subscription is solely for the Mobile App access and does not include the Browser Extension for your PC.
If you purchase a $9 per month Pro License for your PC it will include the Unlimited Mobile access for the app, and you can get this license for $4.50 per month if you have less than 1,000 subscribers. But in order to get access to both the PC and Mobile Licenses you will need to have the Pro license.