Make sure net.ipv4.ip_forward=1 and net.ipv4.conf.all.proxy_arp=1 are enabled and net.ipv4.conf.all.rp_filter=0 is off.
Inbound Interface: Internet facing interface Translations Address: Local LAN IP Protocol: ipip Src Address: Remote server
The tunnel appears to become unreachable after a certain amount of time when no data is being passed between the ends. Any iptables forward rules will not reach the destination in this case. I could not find any information about setting a keepalive variable in the interfaces file for Debian. So, the easiest thing is to setup a systemd service with a ping interval of 5 seconds.