SIM card at & t purchased in the store "7 Elevan" is not activated
I have a Xiaomi Mi9 smartphone that used to work on the t-mobile network. The other day I bought an AT&T sim card for $ 10 in 7 Elevan store. Along with the sim card, there is an instruction on how to activate and select a tariff. The problem is that the network is not available with this sim card and I cannot activate this sim card in any way, on another phone that works with the AT & t network with this sim card the same problem. The store at & could not help me, or maybe they did not understand me.
I just want to understand is this a problem with this sim card? and what should I do with her?