next page in post

I am new to word press. So know nothing!!!
My husband has cutomised a theme for my new style website, so I can go interactive and post articles for comments.

I have writen a post – a lengthy one that I want to split into sections or pages.

I have searched wordpress info to try and do this, recommended to insert <!–nextpage–> at the appropriate point in the post to split the post into pages.
I have done this, but I can only see the first section.
There should be at the bottom of the page a link – page 1,2,3 etc to see the next pages of the post, but this is missing so I cannot see the next part of the post.

How can I resolve this please?

Thanks