If I buy an iPhone 8 plus without a carrier from the online apple store, am I able to add my sim card into that device, maintain my plan, continue text, call, and use data?
Yes, but be careful make sure you get an actual unlocked one From Apple and not one from a 3rd party. 3rd party one are subjected to Reseller Flex policy which will nip you in the bottom.
Retrieving data ...
We're here to help