[NATHELPER] Dealing CGN Public 1:1 mode from ISP

Hi Folks,
I got a new FTTH connection from a relatively new ISP and I am having 
trouble rewriting headers with nathelper.
I am not sure if this is the right mailing list to post this to, but 
here it goes.

My ISP offers static IPv4 behind cgnat, where I am assigned a DHCP lease 
in the 100.64.0.0/10 range, and they do their own NAT which just ends up 
being a 1-1 port map of the external ports to my internal IP.

I know that if I SNAT to the public IP my packets get rewritten 
correctly but my ISP firewall drops the packet, I know I have a static 
ipv4 and incoming connections work and are on the right ports and the 
same with outgoing connections. I also do know that they do not rewrite 
the packets for me and I wouldnt trust them to do that.

I do not know which would be a good way to make it Rewrite the header to 
my public IP, but keep the interface IP as source IP.
I am not sure if anyone else uses this configuration for public IPs 
except my ISP,  not sure how useful it would be to add the capability if 
its not already included.

I would love some guidance on how to rewrite the contact headers in the 
packets while maintaining the Source Address the same