I’m hoping that the OP’s situation has now long been sorted out, but I thought I’d just add that I’ve been having the same thing from time to time (not every month, bizarrely) and I believe it’s a Zwift/payment service snafu and is almost certainly unrelated to OP’s Elite trainer and an actual trial.
As @Gerrie_Delport_ODZ pointed out, @Aoi_Niigaki’s instruction to ‘restore purchase’ clears this up for me
I run Zwift on an iPad (+Wahoo KICKR trainer) and have had my Zwift subscription payment going through Apple/iTunes since I joined in spring 2019. Every two or three months, on the payment renewal date, I have that illustrated Zwift Unlimited message appear when I log in to ride. I’ve always clicked on ‘restore purchase’ and been good to go. Last week, I logged in seconds from the start of a group ride, ignored the Zwift Unlimited message due to lack of time, and then had the trial expired message at the bottom of the screen throughout the ride. At the end of the ride, everything saved okay (which I had been worried about). I then logged back in, restored the purchase, and all was fine once again.
No idea why the sync with Apple payments does not occur automatically every month.