Simulating access from outside LAN / Routing over external computer. How?

I have a Server and a Client for some software (e.g. think of a MailClient and
a MailServer or SSH Server and SSH Client).

Now I want to test if both work together successfully. I can test it
if the two components are on the same computer or on two computers in
the same LAN. Everything is fine in this scenario.

Now I want to test if the access from Client to Server works if the
client is OUTSIDE (!!!) the LAN (somewhere in the Internet).

Now it becomes difficult because there are at least two routers (with NAT forwarding !!)
and some firewalls (hardware and software) between the client and the server.
I want to test now if the routing and firewall filters and NAT forwarding works as well.

However I do not have a computer outside my network resp. it is to
inconvenient to setup such a computer frequently. Hence I would appreciate something
like a 3rd party server service. This service should offer a pure reply/redirecting
of all incoming traffic to the predefined IP/port back on my home address.

Schematically it should go like this:

comp1 (192.168.1.200) ->firewalls->routers(80.230.456.123)->
server mirror->
(80.230.456.123)routers->firewalls-> (192.168.1.210)comp2

Is there something like that or any other solution to simulate access from outside?

Sergej