Ok so let me get straight to the point. I have a 4g connection that I want to run through a vpn that will allow me to port forward and apply a little but of magic sauce that will trick my service provider into thinking that none of the data used is “hotspot” data. The magic sauce has proven successful for quite some time on cell phone and now that i have a 4g modem i want to make a small Linux box (rpi 3) that will also give me some open ports for game servers ect.
so first the magic sauce is a simple ipv4 ttl change to make the ISP think all the data is originating from my “cell phone”. The main part being a sysctl change that will need to be applied at boot.
pkexec sysctl net.ipv4.ip_default_ttl=66 && echo 66 | pkexec tee /proc/sys/net/ipv4/ip_default_ttl
so here’s the HW that im working with
##ISP modem (static ipv4 address 192.168.8.1/24) ##
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VPN gateway a raspberry pi3 with additional USB Ethernet adapter (eth1 is connected to ISP 192.168.8.2/24 also needs to have ttl mod applied to this interface)–> (openVPN connection tun0)–> (UFW or Iptables firewall that will leave open a select few ports)–> (eth0 with either a DHCP server or static)
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##home router / network##
I am fairly familiar with UFW and other ubuntu based networking tools but I just don’t know how to set up this routing scheme. Pretty sure most people on this forum could make this with their eyes closed, if you guys don’t want to write out a full step by step process, you could just point me in the right direction that would be great 
thanks