LTE5398-M904 VoLTE not working

Hello,

I am trying to configure an LTE5398-M904, but VoLTE is not working (Network Operator: GR Cosmote).

VoLTE is enabled on my SIM card/ mobile number and it does work when using the SIM card both in a mobile phone and also in a different manufacturer's 4G VoLTE Router, all test are done in the same place.

But when using the LTE5398-M904 neither incoming nor outgoing calls are working.

I have selected option Voice Service: VoLTE Service , under menu Voice—> Voice Mode.

Also under menu Network Setting—> Broadband—> Cellular Band—> Access Technology I have selected option Preferred Access Technology: 4G , just in case it had any effect, which it had not.

I also want to mention that under menu Voice—> Phone , Region Setting has only USA option available and selected. Option Call Service mode lists Europe Type and USA Type options and Europe Type is selected.

Firmware version is V1.00(ABQV.4)C0

Module SW Version
EG18EAPAR01A08M4G

Can you please help me activate VoLTE? The routers I bought are useless for my use case if VoLTE is not working.