As a quick recap, IPv4 is the older IP addressing standard, while IPv6 is newer. Currently all clients in the internet have an IPv4 address and thus can access other services offered with (public) IPv4 addresses. IPv6 is finally on the rise, but it runs parallel to IPv4 and is incompatible.
UPC offers either IPv6 DS-lite (which means you will have a public IPv6 address, but not have a public IPv4 address and can’t offer services to IPv4 clients), or “IPv4 only”, which means you can offer your services to all clients in the internet.
So if you want to host some services, it’s usually advisable to switch to IPv4 with UPC. The best solution would be a full dual-stack solution, but UPC isn’t sadly offering this for private customers.