I have a Raspbian box that I use for a site-to-site VPN tunnel. It gets internet via a lte modem and PPPD and the VPN is over OpenVPN.
I have not made any changes to iptables except to add a 1: 1 NAT over the tun0 interface. I also enabled IPv4 forwarding (net.ipv4.ip_forward).
My question is: Apart from access via the VPN module, is there any potential for users to access systems on the network using the lte adapter?