Hello,
We have studied subscription plugin code to find out why it does not work for you and found that they do not allow overwriting default templates. We have contacted plugin author https://prnt.sc/1X5ztYYkU-5v and they told us that improvement is added to the to-do-list https://prnt.sc/oCy0Bx9tVfrf
But we don’t have information about when it will be fixed. So, unfortunately, you can use only default emails for the subscription till the plugin author fix the problem.
Regards