netsync with port forwarding

[Hendrik Boom]

I've been using montone for years, and circumstances have forced me to move my 
server behind a Network Address Translation VDSL modem, setting up port 
forwarding to make it accessible to the world.

I use usher.

It's not working.

I have a VDSL modem that is supposed to do network address translation and 
port forwarding.

The VDSL modem is configured to map port 4691 of public IP number 
69.165.134.134 to port 4691 of local IP number 192.168.1.19

Mind you, the configuration menu on that modem has a huge list of 
programs it might be called on to do NAT translation for (includeing 
things like HTML, SMTP, and a lot of games), but monotone's netsync is 
no on the list, so I had to specify the IP number explicitly.

The port forwarding for http, smtp, and ssh work properly.

(1) laptop at home, connected within LAN, so no port forwarding is needed.

    mtn sync mtn://192.168.1.19:4691/rackettown

Works flawlessly.  Also works without the ':4691' (or course)

(2) laptop at home, connection to the public IP address of mmy LAN

    mtn  mtn sync mtn://69.165.131.134:4691/rackettown

fails:
    mtn: warning: no branch pattern found in URI, will try to use suitable 
database defaults if available
    mtn: connecting to 'mtn://69.165.131.134:4691/rackettown'
    mtn:   include pattern  'com.pooq.hendrik.free.fun.rackettown*'
    mtn:   exclude pattern  ''
    mtn: network error: failed to connect: Connection refused

It doesn't help to use 'topoi.pooq.com' instead of 69.165.131.134 (not 
that I expected it to).

(3) laptop elsewhere (using a coffee shop's wifi, to be specific)

Completely unable to make a connection.

hendrik@midwinter:~/dv/fun/rackettown$ mtn sync
enter passphrase for key ID [hendrik@midwinter.topoi.pooq.com] (a2c97968...): 
mtn: connecting to 'mtn://topoi.pooq.com/rackettown'
mtn:   include pattern  'com.pooq.hendrik.free.fun.rackettown*'
mtn:   exclude pattern  ''
mtn: network error: failed to connect: Connection timed out
hendrik@midwinter:~/dv/fun/rackettown$ 


Now ... is there something I should know about using port forwarding with the 
netsync protocol?  Is there something the modem/router needs to know about it?

Or is here some other way of achieving the same result -- letting 
netsync work when I'm not at home?

-- hendrik