How do you switch between classes?
Solution 1:
You don't have to check boxes of all soldiers, you actually choose assault teams, not soldiers, so if you tick infantry, paratrooper and tanker you will queue to a match which has infantry teams, paratrooper teams and tank teams (dependent on your tanker's equipped tanks).
To switch soldiers during battle: when you die and you are in respawn screen hover bottom left to show a button to switch soldiers. In bottom right you can change ATs