NAT Loopback (Hairpin) Privacy

I'm wondering how private is the loopback connection between two LAN devices via router that is ISP WAN connected. Would the ISP pick up a single packet in the traffic, even when the route between the two LAN devices that involves the external IP address (e.g. DDNS and router)?

I'm exploring ways to privately connect from a mobile LAN device to a LAN device by avoiding typing in "192.168.0.XX." I prefer use LetsCrypt-certified DDNS with a wildcarded subdomain and reverse …

nat privacy