Quickly Repost for Instagram- I Can't Cancel my Subsciption After 3 Day Free Trial Period

I downloaded the App, "Quickly Repost for Instagram" (via my iPhone) and signed up for a 3 day, free trial subscription. However I can't find out how or where to cancel my subscription. I tried on the App to contact them and it said, "Error. Please add an email account to Mail app to send email".

I don't have a Mail App downloaded on my iPhone.

The subscription fee is due today and it is way over my budget. I am extremely stressed and have being going in circles trying to find a way to cancel this App, without any success.

Please urgently assist me.

Thanking you in advance for your invaluable help.

Best wishes, Ripke.