LB2120 Configuration behing a Livebox 5 Fiber

Hi Community,

I come here to get your help and support.
I'm computer engineer and normally used to doing such Gateways and router configuration but here I'm facing a problem with this LB 2120 4G Router.

First I tested the proper behavior of the the 4G connexion.

I have set my LB2120 manully in 4G Mode
My PC is connected to my network switch.
The LB2120 is connected to the network switch.

The LB2120 router network address is set as the gateway for the PC ethernet network definition,

All is fine and I can access fully internet pages.

Currently I write this memo beeing set in this mode

Then the problems

1) if I have my Livebox set on the WAN port and the fiber properly connected to the external newtork
The model is set in wired mode
Then I have no access to internet,
I can ping my Livebox I can't see the configuration menu

The LB2120 tells me that it is properly activated in wired mode.

2) Keepalive function doesn't work
The keepalive is seet to ping 9.9.9.9 and 1.1.1.1.

If I unplug the fiber cable, waithing several minutes the LB2120 dosesn't switch to 4G
If I unplug the Ethernet cable then the LB 2120 get switched to 4G

Why doesn't the keepalive function detect that the Worldwide DNS are not accessible to fore the switch

3) SMS
I set the SMS number as (033) 6XXYYZZTT, the router replaced that by 0336XXYYZZTT

All looks fine from a number perspective.

But they when the LB2120 as to send a SMS message it fails to do so showing a error in sending the SMS.

What could be the reason?

If someone has expirience about such Libebowx/LB2120 configuration, clearly, it would save my life and avoid me to send back the LB2120 to the reseller

Thanks a lot by advance