hello stefantasovac
Welcome to the community.
The Connect Box 3 HFC can run in Router Mode or Modem Mode. It always acts as a modem no matter what.
To activate Modem Mode your connection needs to optain a public ipv4 address. You can call 0800 707 707 and request the change to ipv4. Maybe @Daniele_Sunrise will change it for you.
Almost any router can be used with the Connect Box 3 HFC.
There are no other compatible modems for a HFC connection, just the Connect Box 3.