It would be nice to know who your provider is. All others that I am aware of that have made the switch so far also offered IPv4 addresses
to existing customers upon request.
As for VPN, there are basically three ways I know of to implement that:
1) Embedded device: this is the easiest method and the one I use. Some routers offer this, such as the Fritz!Box, Draytek,
etc. If you get a router that contains a VPN server, this is the most elegant version. However, you will find inexpensive stand-alone
boxes that provide VPN from all major companies (I would think Netgear, etc. all still offer them)
2) You can setup your own VPN server at home on an old computer. The most popular server software is OpenVPN
https://openvpn.net/3) You can use a hosted VPN service. OpenVPN offers one, but there are many on the market.
Just note one caveat: you must somehow connect the control side's RRC to the VPN, which can be tricky.
Here again is the embedded device the easiest, because the control side box calls the radio side box and connects the two in a virtual network.
Then you use the internal IPv4 address as the SIP contact to connect.
If you use a software method through a OpenVPN server or use a hosted service, you likely have to use a software client on a PC on the control side,
then connect the control RRC's ethernet to an ethernet port on that PC and share the internet connection.
As you see, just getting an pubic IPv4 address would be easiest, or change your provider if possible. Otherwise, you will have to go the VPN route.
73,
Mitch DJ0QN / K7DX