We have a Gateway router.
I donโt have any personal experience with trying to use a sim designated for another device on the router. From browsing the web I see mixed results. It also seems the cell companies are getting better at stoping this.
I do have some experience with the Gateway. First, it doesnโt seem that it is set up to work with Spring or T-Mobile. Second, I had a lot of trouble getting it to work with both Winegard and AT&T. The problem always seemed to be on their end and I spent a lot of time on the phone while they tried to figure out the reason I was getting 3G instead of 4 G speed. In both cases they finally got it to work and it worked well.
As for plans Winegardโs is very expensive. AT&T prepaid, however, offers a plan where you pay $300 ( plus tax) for one year of service giving you 20 GB per month with one month rollover of unused data. We are currently using that with our Gateway and are pretty happy with it as a alternative for out main T-mobile phone plans