Hi @beatrizs !
Unfortunately, the OPKG feeds do not have a lot of packages. The best approach here is building the entire image using Yocto and then adding the packages you want into this image.
To build a Yocto image please follow the instructions provided here. To add new packages into your image, I recommend checking this question on our community.
By default, the tdx-reference-multimedia-image already has some QT packages, so I recommend you build this one.
Please let me know if you have any issues in the process.
Best regards,
Daniel Morais