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Setting up Cellular Modem between PLC
I want to establish link between two PLC's (Intra-Link LC3000) using a cellular modem sierra wireless GX440 modem 4G LTE.

I currently have this network topology.

Host Machine:
USFilter Intra-link LC3000 (IP:
Cisco switch (
Juniper NetScreen-25 firewall ( which is connected to DSL line.

Remote site 1
USFilter DI (
USFilter DO (
Cisco Switch

Remote site 2
USFilter DI (
USFilter DO (
Cisco Switch

Initially, these two remote units were connected to Host PLC through Trango 900Mhz radios. But due to frequency interferences, we are going with cellular modem. We got 5 remote sites and so got 5 cellur modem Sierra wireless modem GX440 4G LTE for remote sites.

what would be the best way to set up the link between them
1) Port forwarding
2) VPN

and if anybody has done it before, how to go about it.