After removing the snap that was succesfully installed, it would reinstall offline. The ‘core’ snap was missing. (although i had manually installed the ubuntu-core snap, as suggested by snap documentation.)
Trying it now on a physical machine
edit: installing core first solves the problem:
$ sudo snap download core
$ sudo snap download lxd
and then on the offline device:
$ sudo snap ack /path/to/core\*assert
$ sudo snap install /path/to/core\*snap
$ sudo snap ack /path/to/lxd\*assert
$ sudo snap install /path/to/lxd\*snap
succesfully installs withouth network connection.
Note: the core snap is a prerequisite for every snap application.
Thanks for the help!