@Checkdigit 

Hi, welcome to the community.

Deco PX50 only has Ethernet ports and will work well as a wireless router, but not a fiber/cable/DSL modem.

So it could only partially replace the old modem-router combo and you might still need to keep the old modem router or put it into bridge mode.

If the old modem router has gigabit ports, as mentioned earlier, you could put it into bridge modem mode, then the performance of Deco would not be limited.

Thank you very much.

Best regards.