I am in the UK and we have FTTP (Fibre to the Premises) with Zen Internet. 
We also have digital voice - i.e. a landline.

I recently replaced my ISP-provided Fritzbox with a Deco X55 (hardware version unknown but was purchased January 2026 from Amazon.

The Fritzbox had a dedicated port for connection of the landline phone but the Deco does not. 

I've tried connecting the RJ-11 port on the phone to an Ethernet port but am not getting a connection. 

Is there any guidance anyone could provide on this please?