Same here. Last billing cycle (data) Dec 24 - Jan 23 (2019) my data spiked from a normal 300 GB per month to 895 GB. I have a TP-Link modem and router, this is the same set-up for the past few years. My router does not offer bandwidth usage logging. No new hardware has been added for at least the last 6 months. Just yesterday alone I used 100 GB.
Devices: (3) Xbox One, (2) Google Mini, (1) Google Home Hub, (2) Chromecast, up to (4) phones depending on when my children are here, and (4) smart bulbs (TP-Link LB100). These are the same devices I have had for months.