Quality streaming video (such as the ROKU does), has demanding needs of your internet/ISP connection. And the style of internet connection (wireless pickup, likely from a cell phone tower) you have is usually not up to the challenge of providing the consistent internet bandwidth the ROKU needs. If it were me I wouldn't even try, as the results of getting it to work with your existing internet (if you can get it to work at all), likely "wouldn't be pretty".
In THEORY it is possible to overcome all of the problems, but the reality is that you will likely get "bit" by at least one of these problem (below), and all of them need to be "solved" to get things working as you want:
1) Your internet connection is USB, which is not a connection type the ROKU box supports. So you will have to use something else to convert the USB internet connection to either ethernet or home wireless (which the ROKU supports). As mentioned by other posters, a PC running "internet connection sharing" might be the easiest solution. However, I think there are also some stand alone routers with the ability to convert USB internet to normal ethernet and/or wireless (I'm not sure about costs and exact router models, however, so you would have to research this before buying).
2) You need a fast enough internet connection for video streaming. Many cell phone type modems (which is I think what you have), don't consistently have enough speed to support the bandwidth needed for (decent quality) real time video. So you need to know ahead of time how fast your ISP plan is realistically going to be (not the "advertised speed", the realistic testing of actual speed you can get). Ideally, this is 5 megabits or higher, but even a 1.5 megabit download speed could be doable if you are willing to put up with lower video quality. However, slower than that, and the video buffering would just be way too painful to watch.
3) Some of these ISP contracts have TOS (contract rules) preventing either such sustained video streaming, and/or the "tethering" needed to make that streaming work with a ROKU. So you need to very carefully check your internet service provider contract, to make sure that this is even allowed!
4) And finally (and perhaps most importantly), many cell phone type internet plans have very low limits on allowed usage in a month. For example 5 gigabytes a month limit is VERY COMMON with cell data plans (and some cell data plans have even lower limits than that). With the ROKU often using as much as 2 gigabytes of data PER MOVIE YOU WATCH, you could blow through a monthly data limit that low in under a day's use of the ROKU! And once you blow through that limit, you will (depending upon the details of your contract) either: A) have your access "throttled" to a practically unusable speed for the remainder of the month, B) have your plan terminated, or C) have a VERY COSTLY "overage" charge added for your use the rest of the month. So if you are going to use a ROKU on a cell plan (which appears to be what you are asking about), you have to pay very close attention to the bandwidth used by the ROKU (with your viewing pattern) and how that bandwidth compares with the data limits built into your ISP plan!!!