MS VPN on NT 4.0 wont'

It's a Cisco 1720. We purchased the VPN hardware option and
we have some problem using the VPN with LAN Modem (3 com 3C886) at
the remote office is Costa Rica, Penang. IT seems like the NAT on LAN modem
will not work with the VPN IP SEC.

Here's what I've done so far.

1) I setup a test with static IP addresses and LAN modem.
I setup so that each PC and LAN modem has it's own Public IP addresses.
I setup a range of 8 IP addresses and 1 dial-up account.
It works without VPN but with the VPN only 1 connection allowed. When the second PC tried to connect, it fails.

2)I also tried Microsoft VPN (PPTP),
With the phone line, it also only allow 1 connection at a time.
With the LAN modem, it ofcourse allow 1 connection only.
There could be some setting that I need to verify.

3)I did some research on how to make this work with Cisco equipment.
If we could just forget about the LAN modem and replace it with something else.
It require a router for each office and also the VPN hardware option.
And this will push the cost to $1500/office which I don't expect.
They also said instead of us purchased VPN hardware for 1720, we could have gone with
the Cisco concentrator 3000 which will work with NAT. OH, well it's toolate for that.

At this point, I would like the remote office to try using MS PPTP and see if it works with
the LAN modem. But for now, I need some help to set it up so it will allow more than 1 connection using the phone line.