I'm not the most familiar with the D-Link routers Verizon hands out, but what you're looking to find is some information on the WAN or "Internet" end of the modem for how many bytes of traffic you have sent and received. This information may be for when the router was last rebooted or it may be from the last time your connection re-connedcted if it has dropped, so it isn't really accurate when it comes down to a breakdown of things. If you can get a byte count, it is easy to convert to Megabytes or Gigabytes from that point (Factorsof 10 in a general sense). By definition a Kilobyte is 1024 Bytes, but in some cases a factor of 10 is applied so a Kilobyte is 1000 Bytes. A Gigabyte can be 1024 or 1000 Kilobytes depending on how the data is being measured like previously mentioned.
So, 40,960 Bytes / 1024 = 40 Kilobytes / 1024 = 0.039 Gigabytes.
Considering how large (or small) the data plans tend to be on a Mi-Fi device, consider the type of network traffic you currently have and gauge it against future use. If you currently have gamers in the house, a Mi-Fi should already be off the table as a full blown replacement as cellular connections are poor quality connections for playing video games on. Not to mention, games and patches can consume the cap quickly if the gamer has a lot of games that they play. Also, if your home is Netflix or YouTube heavy, without turning down the video quality you'll find it's quite easy to run over a cap. On a Mi-Fi myself, I have to restrict my YouTube access since I am picky about low quality video, and force everything to HD. I can slip in a few videos on a Grandfathered 3GB plan with Verizon but if I watch some of the common, 20 minute videos I watch, that's a Gigabyte per video on average. My normal net usage would burn me through the cap in less than an hour.
Just some food for thought, but keep in mind Verizon will most likely be unable to provide any sort of data usage counter. You may need to use third party routers or software in order to determine your usage per month.
