Have you since been able to play any content?
Assuming your videos do not include Dolby Digital, DTS, DTS-HD, or True-HD audio (which Infuse must pay royalties for and are therefore limited to Pro-subscribers), I’m not aware of any other codec-related restrictions — though iirc, I originally trialed the Pro Version of Infuse and happily bought it at the end of my trial — so there might be some I’m not aware of.
That said, I’ve yet to read any support requests in this forum from a free user who couldn’t get media to playback (because they were a free user) that wasn’t related to one of the above listed Pro-only audio codecs.
If you are still unable to play any content, and still receiving prompts to subscribe, I’d suggest deleting the app and any data it might have saved in your iCloud account, if enabled) and reinstalling it.
If you trial the Pro-Version, you will be prompted to activate the premium audio codecs (there will be two separate prompts) only when you first attempt to play content that relies on those codecs.
If you are a subscribed Pro user, there will be no additional fees incurred (despite the prompt saying something like “free purchase, you won’t be charged” but still using the purchase interface) — this is newly required because Infuse now pays individually for each user who activates the premium codecs.
If you are on a trial subscription and encounter one of those prompts, I don’t believe you’ll need to actually purchase anything either — but I’m not certain so read the prompts carefully.
Infuse is an honest broker; they don’t play bait and switch games or hit you up for in-app purchases to keep doing whatever you were already doing. You pay a small subscription fee to access Pro (I pay annually) and that is all. As long as I have an active subscription, I’ll always receive every minor and major update going forward.
Sometimes popular, oft-requested new features or feature upgrades Infuse reports they are working on take a bit longer to arrive than originally anticipated, but that’s about the worst of it.