There is nothing that meets your requirements. There couldn't be
because of the nature of Jet -- it can't work via a non-SMB
networking interface, except for transferring files. Internet
replication does that over FTP, but at the cost of including a
dependency on IIS on all machines on each end of the synch (that
means IIS running on all your laptops that need to indirect synch
from the field).

The only solution is indirect replication over VPNs. This works just
fine over dialup, as the bandwidth required is very small.

But that requires programming and significant infrastructure setup.
If you can't afford to pay someone for that, then replication is not
the solution.

If you've got dialup connections, you might consider hosting your
app at a central location on a Windows Terminal Server, which will
be fast enough over dialup (and can be speeded up by purchasing some
of the Citrix extensions).