I've been using Project Fi as my mobile carrier for a little over a year now and only recently found out that I can order additional data-only sims. There's no extra charge and they just use data from my plan as I go. I've been curious about the 3g/4g backup WAN option on the Zywall USG series (I'm using a USG 40) but the list of compatible modems is an image (really Zyxel?) an only lists brands and models with no useful carrier info. After dealing with support I also found out that this list does not apply to all USG models, the D-link DWM-157 which I tried first does not work with the USG 40, even though it does work with project Fi, at least with 2g access.
The data-only sims work with T-Mobile's network only, unlike the compatible phones which also use Sprint and US Cellular to round out the coverage map. I finally settled on the Huawei E397, also sold as as the Boltz modem for Cricket's service. This same setup should work with either Cricket or Ting for carriers if you don't have Fi. To order a data only sim, log in to your project fi page on a web browser (the option doesn't show up on the Android app) click on 'manage plan' and 'Add data-only SIM'
To make this all work, you'll need:
