tablet that will work with sim card

Is there a tablet 7-10" of size that will work with at&t/t-mobile or any sim card if i have unlimited data plan from i can browse even if im on the road.

i found huawei s7 and dell steak 7.

are they unlocked,???are there any others?? can any sim card be used in these??
my budget is under $300.
please help and recommend a good tablet that can work wity my current phone sim card.

much appreciated.