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Is DMZ a solution for PPTP server behind NAT?

Quick Facts:

ISP Name: PCCW Hong Kong

ISP Service: FTTH

Modem: Huawei Echolife HG8045

Router: Huawei Echolife HG8045

PPTP Server: Linksys RV042

Config: ISP->Fiber->HG8045---->Internet TV Box



................................|------->Homeplug AV2 bridge->802.11g AP

Hello! I am from Hong Kong (sorry for my English ). I signed up for the PCCW FTTH service earlier this month but I found out that the modem they provided is a Huawei HG8045 GPON terminal which is a modem router combo.

The problem is that my ISP wont provide admin access to HG8045's web administration interface, I can only tinker some minor settings e.g. port forward, DMZ, wifi ssid, wifi security..etc, using an "user" account which I obviously cant disable NAT and change that to a bridge nor enabling PPTP passthough.

The old Linksys RV042 comes handy to bypass internet block at workplace or public wifi AP with PPTP server VPN function but I it barely works behind NAT, I can never connect to the pptp server at work, I tried forwarding port 1723 but it doesnt help.

I googled a bit and found that pptp uses GRE encapsulation which is not bound to a port and my only last hope is to assign my RV042's IP to DMZ, though it sort of worked as I can establish a VPN connection, got a valid ip from the RV042 and the RV042 administration interface could be accessed, strange that I cant visit any other webpages through PPTP tunnel.

Before troubleshooting other possible problems i.e. DNS setting, I would like to know if DMZ is a viable solution to pptp server behind NAT? Does it solves the GRE problem?