Ok, so this is a big network issues then,
dependent on the routers and how you want and have things talking to each other.
*you could use iptables(nat), or DMZ
when i have two daisy chained i usually separate the two routes subnet via dmz
so the main "isp" modem connects to first router(could be builtin), i dmz the last ip (ie 192.168.1.254) and the next router is set statically to the dmz (this way all other traffic from the first router is unable to talk to the DMZ..., all ports are open and now assigned to the next router for port forwards.
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*easier to setup, but may stop communication to the server form router 1
so, i would recommend this setup, assuming that this will not impede what you require form the server PC.
internet > ISP modem > Router(builtin?) *dmz ip > *(static to dmz ip) router 2 > server PC
port forward required port in router 2, for access to hfs.
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if DMZ is not what you want as it can impede on router 1 communication to server..., you will have to define ports to the router ( NAT/iptables setup)
(a pc on the internet will connect to port 80 to router 1), a lan pc will connect to port 5001 on router 2)
example setup defined ips:
router 1 lan ip: 192.168.1.1
router 2 public ip: 192.1698.1.254 > lan ip: 10.0.0.1
server ip: 10.0.0.254
hfs port set to 5001
internet > ISP Modem > Router1(can be build in) > router 2 > server pc
*HFS active and setup to port 5001 server is set to a static IP 10.0.0.254
source ip 10.0.0.254:50001 Website to a client pc connected to router 2 should be able to access hfs on port 5001
* Router 2, port forwards: starting port 5001 to ending port 5001
server should be accessible from router 2 lan via 10.0.0.254:5001 and available on router 1 via router 2 public ip of 192.168.1.254
* Router 1 port forwards: starting port 80 to ending port 50001
source ip 192.168.1.254:50001 Website to a a client pc connected to router 1 should be able to access hfs on port 5001
Pc on the internet to connect to your server
A pc outside your network on the internet should be able to access the HFS site via port 80 and your public ip: x.x.x.x:80
*(for simplicity if your ISP blocks ports, then change start port 80 to 5001 for both the start and ending ports as in router 2!...)