I have had this app on my phone and tablet for a few years now and I wanted to be able to watch it on my Roku, BUT, I can't even do that because Roku wants me to pay a subscription and I REFUSE to do that if I already have one! You should give a choice to sign in as well as sign up for this app and I find it disappointing that you don't even give that either!!!!
Things like this are controlled by the channel/content providers. (All of the channels I use allow multiple devices and you can just log-in on the Roku.)
I googled and found this:
https://passionflix-support.maginepro.com/support/solutions/articles/35000079693-passionflix-on-roku
I notice on that page there is a link to submit a new ticket. That might be the next thing to try.