Max Nowack

This plugin can display more than one page on a page.

ShowMultiplePages

Yannick Lefebvre

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site. The number of times that these can load is configurable. They can load content from external sites or custom HTML code into the dialog. This plugin can be used to invite people to register to a newsletter, respond to a survey, or simply welcome come to a site upon their first visit.
You can try it out in a temporary copy of WordPress <a href="https://demo.tastewp.com/modal-dialog" rel="nofollow ugc">here</a>.

Modal Dialog

The others either didn't work or caused massive JavaScript problems, breaking other plugins.

This plugin worked the best.

Worked better than the others

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site.

ShowMultiplePages vs Modal Dialog