You will be able to use the 3G and 4G if only you will have 4G plan and obviously the coverage.
I believe Samsung S5 is 4G capable, because this is the main problem, not the backward (3G) compatibility.
As to the other question your understanding is not correct.
It seem you are a not a technical person, so I will spare you explanations. It should be enough when I say that among many the authentication keys are different.