No bars when inserting new ATT tablet sim

I just got a Straight Talk "Bring You Own Tablet" sim card kit containing 4 sim cards (2 t-mobile, 2 att). When putting the nano sim cards in my Ipad mini with LTE, I get no signal bars for ATT or T-mobile even though both carriers have a strong signal at my location. Instead there is just an "X" showing no service. I have not yet activated any of these sims to a Straight Talk tablet plan. With most other carriers, I get a signal even before activation of the sim. Is the situation different for Straight Talk in that I need to add a plan to see signal bars??