Don’t do that. In LO, any shape can contain text. So, label your shape as this is easier than to create a rounded rectangle and a text box you must align with the rectangle. And once again, it is easier to do it with Draw.
Using text boxes for that is faulty because you disconnect the text inside from the flow, preventing Writer to layout the text globally. Notably, you can’t style with Heading n to tell Writer what is a heading and what is not.
- Either use a paragraph border
Only borders around identically styled paragraphs can be merged into a single block. It is easier to achieve with paragraph styles than with direct formatting
- Or create a section for the block and give a background colour
A section cant have a border but may contain several differently styled paragraphs which will shared the section background
Unfortunately, paragraph border can’t be rounded for the time being but I seem to have read somewhere it is on the way for a future release.
Also, if your document is single page where you have unrelated “areas” for various topics, then Writer is probably not the ideal tool. Writer is flow-oriented where pages are allocated on demand to accommodate text size. Pages make up a sequence where text real estate is managed in a continuous way. To handle the case where you split your page into “areas” with fixed size (no automatic allocation to expand them) you should look at applications like Scribus.