The podCIDR field of the node is only set if kube-controller-manager is configured to manage pod addresses via --allocate-node-cidrs=true. Since Charmed Kubernetes relies on a CNI to manage pod addresses, that field isn’t set. In general, it’s not really a reliable field and there’s a proposal to remove it entirely, so it’s unclear to me why the LB implementation you’re using would depend on it being set.
What LB are you working with specifically? Perhaps there’s a way to inform it about whichever CNI you’re using and have it get the CIDR from that instead.