If you install it directly in sys-net, it’ll be gone by next VM restart. If you want it to be persistent, you need to install it in the template that sys-net is based upon. You can find out which template it is with the following command in dom0:
qvm-prefs sys-net template
If you see it’s a dvm then you need to install it in the base template, not the dvm (for example, if you see debian-11-dvm, you need to install it in debian-11), then shutdown the template and restart sys-net.