ISP is using what is a CGN, major pain in the ass for anyone every where.
Like @starbetrayer say ask your isp for an public isp (if not a business subscription, they probably will say no). Switch ISP but down the line they too will start doing this.
Option two, is get a VPN with a dedicated public ipv4 and pipe all traffic through that VPN.
There is one more option which is IPv6, however pption tee is as of now hard since Plex is lacking proper IPv6 support.
More and more people will get these issues as ISPs are running out of IPv4s and have to resort to CGN or other large scale natting.
I am lucky enough to have my own /26 which is not going away anytime soon.
So this is yet another lead to why Plex must support full ipv6 throughout its ecosystem-
