When I upgraded my nextcloud from 12 to 13 I forgot some apps (calendar, contacts, news and phonesync), so afterwards they obviously didn’t appear installed. Hoping they would discover that there was data in the database and use that, I went to “Apps” found the apps and activated them.Now they’re
there, but the data isn’t 
What can I do (I have a DB backup)?
Nothing has happened that should change anything on disc, so would it work to reinstall the old version (12) with those apps and overwrite the database with the backup, and then re-perform the upgrade? (obviously following all the advice relating to upgrades).
All the data should be back when you enabling the apps again. How did you upgrade? Were there any errors? Except for the missing app data, everything is working?
You can first try to re-run the upgrade command, which should detect if something has not been converted correctly:
sudo -u www-data php /path/to/nextcloud/occ upgrade
(use the webserver user which is www-data per default on Ubuntu/Debian).
Do you find any tables for these apps in your database?
Just forget it. I just forgot to log out from the admin account (which I naturally used to reenable the apps) and into the account that data is on. So everything is working.