Trouble importing external, non-WP, content

I need to import a ton (hundreds and hundreds) of old .htm files and I’m not sure if this is the right plugin to be using.
But even if it is, I’ve been experimenting and I can’t get the plugin to work properly even with test data.
To remove as many variables as I could I exported my posts from WordPress to an wxr file and removed all but one post and replaced the data with some dummy data.
When I try to import it though, the body of the import page just white screens and I can see that a post has been created but it’s just a txt file with none of the content I added.
I’m really confused and I’m not having much luck in my research. Can anyone give me any guidance? Thanks.

https://www.ads-software.com/extend/plugins/wordpress-importer/