A:Answer TL;DR No, don't worry about the SIM card. The tech hasn't been released yet.
Your SIM card won't make a difference what frequency (3G, 4G LTE, or 5G) your phone connects to. That functionality is already on your new phone. The issue lies with 5G coverage right now. 5G is more or less going through a testing/proving phase and isn't widely available. Even where testing IS going on, it may just be commercial. Also the range of the towers is much shorter and there's a lot of misinformation about 5G out there holding up its "release". They expected it to release later this year, but w/ the virus....who knows.