NAT to ISP Modem via OpenVPN

[Gil]

I want to web access the ISP modems on multiple Site to Site routers (OpenVPN).
The issue is that (for ease of installation) each Modem is configured with identical IP addresses and gateways. So routing directly to the modem won't work out.

So, I have a NAT / Port Forward on each client router:

eg: for Router2 -
OpenVPN TCP * * 10.10.2.254 8088 10.0.0.254 80 (HTTP)

This works for some ISP Modems,
but others do not render the web page properly or at all.

Is this a function of the Web Server (eg nginx) within each Modem?
Can I do anything about it?

[Crunk_Bass]

Maybe you need to add an additional outbound NAT rule?
Something like Interface: Modem Interface, Source: OpenVPN Subnet, Destination: Modem Address, NAT Address: Modem Interface Address