How to disable external IP address

I am trying to figure out how you disable access by external IP address, primarily for security purposes. Also it is really a waste of an IP addressas I dont need it. I want to access my Ubuntu droplets/servers via a publicly facing VPN server droplet, then access the subsequent droplet/servers via the local VPC i.e. 10.105.0.1 on VPN server droplet to a service running on 10.105.0.4… my client via VPN can see all of thesejust fine. However, if I apply a firewall to the servers, the configuartion applies itself to both external and VPC connectivity. It gets hard to define rules for this… unless maybe I say you can only accept connectivity from the 10.106.0.0/20 range??? Ideally, rather than create firewall rules on the droplet, I am after a way to do this through DO web UI. However, when I try to disable eth0 on one of the droplets, it isnt present in the /etc/network/interfaces file… It appears I need to run network manager which is a UI based program. Obviooulsy this is not possible over SSH. All very confusing. Surely others must require something similar for DB hosting were they dont want any aspect of it externally reachable, but reachanble on internal private network.