Can't buy prepaid sims or plan online?

Just got T-Mobile home internet and set up My T-Mobile account. Want to purchase prepaid sims for our unlocked iPhones but when I try to shop through my account it will only offer unlimited plans.  Don't need 5G or unlimited data on our phones and their 6GB a month plan would work for us but I'm unable to buy it online...whats the deal with this?