Setapp is a software subscription service which includes Ulysses and over 190 other macOS and iOS apps. All apps in the Setapp Store are fully functional, without any limitations or in-app purchases. You can read all about Setapp on their website.
In order to use Ulysses on your iPhone/iPad via Setapp, you must register your mobile device as an additional device on your plan. Once you’ve done that, you can check out the iOS/iPadOS versions of Ulysses that appear in the “Available on iOS“ category section of the Setapp store. For more information, please check Setapp’s help section.