openwrt fw4 ttl mangling

1.) put this in /etc/config/firewall
config include
    option path '/etc/firewall.user'
    option fw4_compatible '1'

2.) create the file '/etc/firewall.user'

3.) put this line in it:
nft add rule inet fw4 mangle_forward oifname usb0 ip ttl set 65

4.) restart the firewall
/etc/init.d/firewall restart

This sets the TTL to 65 (just a random number I picked) of all packets forwarded out on usb0 interface.