Yellow Guide Lines Disappeared in Pages

In Pages Settings > Rulers, select the following, so you will have dynamic alignment guides as you move objects around in the document.

The alignment guides that you drag out from the rulers are a per-page entity in Word processing or Page Layout documents. Each time you switch to a different page, you must drag the alignment guides from their respective rulers, or give the dynamic guide settings from the image above a try without using the ruler guides.

I would have inserted the line shape and straightened it. As that is done, the dynamic guide will appear to show you the horizontal line you are rotating the line shape too. Then you drag it to lengthen it and the vertical dynamic guide will appear when the line segment is centered on the page.

Next, I insert a text box just large enough for the Times New Roman 48 pt Bold content. As I drag that over the line segment and position it, the dynamic vertical center guide appears to help with that process. I then do a select all and group both items from the Arrange panel. Additional movement towards the top of the page that varies left or right will dynamically show a vertical centered guide.

I saw no need to make a full page sized Text box based on the content you show.