I don't use LibreOffice and, to be honest, I've sometime struggled with using labels in other word processing software, so please forgive me for not answering your question directly. However I wondered if you're aware that you can design your labels online. In my opinion, it can be far easier than using
any word processing program.
If you're interested in giving it a go, start here:
https://www.avery.co.uk/template-j8163
Click the red 'Start Designing' box.
If you see a registration form, you can either fill it in or click on 'Remind me later'.
Click on the label format that you want. (It's likely to be the blank 'landscape' label). Then click on 'Select this design'.
You'll then see just a single label displayed.
With 'Edit all' selected, fill in any information that you want to appear on every label. (Click the 'T' to access the text options. Click 'Add text box' to create a field to type in. Adjust the font and size to your requirements. Drag the box if you want to move it.)
When you've got a 'master' label created, then switch to 'Edit one' to change individual labels).
Then go to 'Preview and print'.
(Sorry if I've wasted your time!)