OK I found a solution in the old forum here and modified it to run the cron job instead. Thanks to @Shigure
First get the pod name
k3s kubectl get pods -A | grep nextcloud
Then run this to add the indices, where nextcloud-abcdef12345-ghijk34 is the pod name got from last command.
k3s kubectl exec -n ix-nextcloud --stdin --tty nextcloud-abcdef12345-ghijk34 -- su -s /bin/bash www-data -c 'php cron.php'
Worked for a couple of days now, it seams to solve this but I would prefer a proper solution without external task to run, and with a lower interval than the minimum of 1h that TrueNAS GUI offer.