Hello guys, here are the last news:
I removed both /e/ and Fairphone OS and tried LineageOS 17 (Unofficial GSI build by fwg-cag).
At first, no Data connection at all, the apps couldn’t connect.
I checked APN settings, they were incoherent with the previous one (and I can confirm that Bouygues Telecom uses IPv6 APN, setting it to IPv4 doesn’t work).
I wanted to save the settings, but mispressed and delete the APN instead…
And, surprise! The only APN I had in the list was replaced by 3 others (Bouygues Pro, Bouygues WAP and Web) and the Data connection went back!
Tried to activate Hotspot and play Dragon Mania on my other device: the game connects. Tried the Store: OK. The browser: OK.
Installed Blokada v5, activated it: Connection still works. Perfect!
So, in conclusion, by repaired my problem totally by chance, and I can enjoy again my connection, fully.
Since I’m using a totally unofficial LineageOS build, I will try the phone features, if something goes wrong I will save APN configurations and re-apply them on Android 10 or /e/.
Thank you again for your help and for the informations. For now this problem is solved.