Well, I solved my problem.
Let me share the solution:
Here is the trick explained on the ISP side:
https://blog.wirelessmoves.com/2020/02/ipv6-only-in-mobile-networks.html
So, this links you to https://en.wikipedia.org/wiki/IPv6_transition_mechanism#464XLAT , and this links to the only CLAT implementation for linux: CLATD : https://github.com/toreanderson/clatd
=> You just need to install clatd on your librem5
Absolutely 0 configuration… Once up and started, you have to wait a few seconds after LTE is up (in IPV6), then you can test with ping 8.8.8.8 for example. (the wait time the 10s delay in the perl script of clatd, waiting for IPv4)
And wifi hotspot is working. (I am writing this on my computer connected to internet via my librem5 with the wifi hotspot).
I can detail install if needed, but it is pretty straightforward and well documented in the https://github.com/toreanderson/clatd readme.
Just look at ip addr for fun… It is magic !
5: wwan0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
link/none
inet6 2a04:cec0:f085:1972:7776:f934:7c6c:f3da/64 scope global noprefixroute
valid_lft forever preferred_lft forever
8: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 00:0e:8e:a3:2e:a6 brd ff:ff:ff:ff:ff:ff
inet 10.42.0.1/24 brd 10.42.0.255 scope global noprefixroute wlan0
valid_lft forever preferred_lft forever
inet6 fe80::cbc:80f7:ca13:552b/64 scope link noprefixroute
valid_lft forever preferred_lft forever
9: clat: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 500
link/none
inet 192.0.0.1/32 scope global clat
valid_lft forever preferred_lft forever
inet6 fe80::f812:9f31:3167:f31f/64 scope link stable-privacy
valid_lft forever preferred_lft forever