@bitfield 

Hi

In theory you could try to use a converter to connect to the Deco, but it's best to get in touch with your internet service provider directly to find out if the converter may take the place of the modem because they might have restrictions on registered devices.
The configuration method is as follows: you could go to Deco APP > More > internet to input internet connection type, username and password.

If configuration fails, it is important to confirm the requirements with your internet service provider, as we can't guarantee compatibility or availability.

  @Joseph-TP 

Thank you for the answer @Joseph-TP. I do have SPF +ONU. When I configured my Deco it was connected to an Ethernet port of my ISP's router. Now I want to connect the fiber to the SPF, the SPF to the ONU and with ethernet cable to connect to the Deco. You suggested to go to Deco APP > More > internet to change the setting, but, it looks like the Internet settings are read-only. I had to reset the device to default and to reconfigure in order to define another internet connection type. Am I missing something? Is there a simple way to do it?

  @ImtiazAli 

Thanks a lot, now I see it...