newsletter – Plugins don’t work on custom theme

I’m trying to add newsletter plugin (mailpoet) to a wordpress site which has theme made by somebody.

It seems like he made this theme, so any new plugin don’t work. In my case – popup from mailerlite doesn’t appears, even in the source code of whole page. But, cookies notification popup which he added works.

When I change theme for some default it works normally, so I think it’s not the case of other plugins interfering.

I need to add mailpoet manually in functions.php? or is it some other way to add it manually? maybe of mailpoet in footer.php?

Code of which file I should provide, so you can help me? functions.php?