On 11 Oct 2021 at 08:43p, Tommi Koivula pondered and said...
 
 TK> His tunnel is working, he can make outbound connections. So the protocal
 TK> 41 is open to the tunnel endpoint computer. (PI).

I can also ping the Debian box via the Pi endpoint tunnel using IPv6... so I figure things must almost be working as I can reach the system from the internet via IPv6 and the tunnel using ping

--- Mystic BBS v1.12 A47 2021/09/29 (Linux/64)