Confused about Data sims not working on different routers

This not so much a complaint (yet) as a query. I tried to upgrade my broadband data contract this week at Vodacom shop in Centurion mall. I was told I could get a 250GB data plan for R249 a month, but I am only able to use that sim card in the supp**** home non-portable router. I am told the sim will not work in a mobile portable router, and if I want a sim for a mobile router, the same amount of data would cost me more than R1000 a month. I am extremely confused by this and would love for Vodacom to explain the logic and reason behind this and if it is indeed true. If its true I think its time for me to explore other service providers.