How does a computer in an IPV4 network communicate with a computer in an IPV6?

My Cell modem only supports IPv6 addressing. I can browse the Internet and access IPv4 addresses using it. However, the destination host is returned as inaccessible when I attempt to ping this IPv6 address on a cell modem from an IPv4 network. I would like clarification on the following queries:

1. Can a PC connected to an IPv4 network—which may have firewalls, NAT, and other security measures—access IPv6 addresses with little to no modification? Despite my attempts, the Teredo service only appeared to function for a short while before I lost the ability to connect with IPv6 addresses. I want to talk to IPv6 directly, so is there a way for me to go beyond the internal network layer?

2. Is it possible for me to use a VNC service, for example, on a computer linked to an IPv6 mobile modem from a standard IPv4 network?

Could you also tell me where I can get some research and understanding of this kind of problem?