Glad to hear you solved this @juliancmg! 
For future reference, in case someone would be interested in the solution, please see the description of the fastlane step:
If `Gemfile` exists in the `work_dir` directory, fastlane will be used by
`bundle install` && `bundle exec`.
Otherwise the system installed `fastlane` version will be used.
This means, that if you use the ruby Gemfile & Gemfile.lock in your repository to specify & install the required fastlane version on your Mac/PC then that will be used by the step automatically and the version specified in the Gemfile.lock will be used.
Otherwise the system installed version will be used, which of course can be used, the same way you would on your Mac/PC:
As mentioned there, if you simply need the latest fastlane version every time the fastlane step has an option for that too 
If anyone would have any questions just let us know!