Comment 12 for bug 1945030

I don't know if this might work for everyone but a workaround to the "Cannot refresh cache whilst offline", is to restart the packagekit service:

Do a:
systemctl status packagekit

and you will get lines like this:

PackageKit[2067]: refresh-cache transaction /193_cdedbaab from uid 1000 finished with failed after 148ms

Then do a:

systemctl restart packagekit

and then:
PackageKit[6339]: get-updates transaction /201_daaaadaa from uid 1000 finished with success after 222ms

which means it works, i was able to install normal packages from discover and apper.
systemctl status packagekit