Set APN protocol for mobile modem

[Edouard Debry <edouard debry gmail com>]

Hello,

I run mobian on a pinephone.

When mobile data is turned on, the phone receives an ipv6 only
address. Trouble is that it seems to break connexion sharing.

Some people solved this issue by setting their APN protocol back to
ipv4. However, I cannot find this option on the gnome user interface,
I can only choose the APN name.

But I am sure I can force the APN protocol by writing it directly to
NetworkManager's configuration files.

Could someone gear me towards the correct configuration file ?

Regards