Because your LMT ISP decided that if client wants their own IP they should upgrade to Business plan, that's all there is to it. They probably don't have enough IPv4 addresses to hand them out to each client and use NAT instead, many many clients under same external IP, so obviously incoming connections gets dropped. I find it extra interesting that they decided to do exactly the same for IPv6 addresses, which surely they have enough of, so most likely business plan upgrade is primary reason here.