You may bring your own Samsung Galaxy S6 and as pointed out by @jben it has the needed slot for a Republic Wireless SIM. That S6 needs to be a specific model (SM-G920T) running one of the following Android operating system builds:
The above infomration is referenced by Republic here: Republic Wireless Phones – Republic Help.
If the S6 you wish to bring is a different model number and/or not running one of the above referenced software builds, it will not work on Republic’s network. Alternatively, look for Samsung’s manufacturer’s SKU SM-G920TZKAXAR as shown on Samsung’s website here: