Hello Tony Vu
the first question to you is - what VBP software version do you use?
Can I put private IP for subscriber IP in VBP with NAT port on modem?
Yes, you can. From v11.2.10 the VBP can be installed behind a perimeter security NAT device. The NAT configuration must be a Public IPv4 address configured for a one-to-one NAT translation without any port-mapping to the VBP-ST internal DMZ Subscriber/WAN IPv4 address.
you can configure NAT settings on VoIP ALG VBP's menu.