International roaming issue : getting IP values but no valid routes

Hi all

I recently use Gl.Inet GL-AR750S mobile router OpenWRT 21.02.0 with
success in Belgium (Telenet operator) and I am getting dual IPv4 and
IPv6 mobile connections.

This week I was in Paris hoping to data roam (EU roaming is free)
using this router.

And, my (Android 11) phone did data roam both IPv4 and IPv6 just fine
(using France operator Bouygues). In fact my old Huawei MiFi Hotspot
also did data roam just fine (using same France operator Bouygues),
albeit only IPv4 (the Huawei MiFi Hotspot does not support IPv6).

The idea was to replace the old Huawei MiFi Hotspot with the new
GL-AR750S OpenWRT.

But, my OpenWRT GL-AR750S had problems to data roam: I did get IPv4
address and gateway and route and I did get IPv6 address and gateway
and route and prefix delegated and the clients got their IPv4 and
their IPv6 parameters, but: no address on the Internet (e.g.
2001:4860:4860::8888) could be pinged (not from the router not from
the clients).

FWIW: roaming is enabled on the GL-AR750S OpenWRT:

+QCFG: "roamservice",2

In short, just like in Belgium:Telenet, in France:Bouygues IP appears
to work, but, unlike in Belgium, when roaming in France there is
nothing reachable on the Internet.

So recall that, in Paris, both Android and Huawei MiFi Hotspot did have
everything reachable on the Internet using similar Telenet SIM (in
fact I did swap SIMS as well) and same APN configuration
( no username no password), and that the GL-AR750S
OpenWRT works just fine in Belgium.

Does this ring a bell?

Also what would be the way to choose another nerwork operator?

Basically I could ask:
"Has anyone successfully mobile roamed in Europe using OpenWRT 21.02.0?".
(perhaps my issue is country and provider specific)
Thanks for any testimonial.