I've been told there's a law that my mobile phone operator has to turn off all firewalling on my connection if I ask.

I don't know about that law, but GP's point was that you don't get a public IP anyway, firewall or not. And with this NAT in place, you can't ask them to forward specific ports to your equipment.

In France, CG-NAT is getting widespread even for fixed, FTTH links. I'm typing this connected to SFR, which provides a static IPv6 /56, but IPv4 is behind CG-NAT. I can't host anything on IPv4. I think there's an option to get a fixed, internet routable address, but not on the "discount" plan I'm on. I hear you maybe can ask support to get you out of CG-NAT, but that doesn't seem very reliable.

Free (local ISP), by default, doesn't give a static IP for fiber, but you can ask for one for free through your online account page (you just need to tick a box).