This works well if you pay attention to the details. There are specific requirements that are listed by Republic Wireless in the following document:
Republic Wireless Phones – Republic Help
In addition, a very helpful member of the Community has written the following guide:
Getting a phone from Motorola or a major retailer like Best Buy or Amazon is likely to go well. Be sure you have a good return policy with whichever company provides the phone you bring.
The BYOP SIM is GSM not CDMA. If you are migrating from one of the older Republic phones (Moto E1, E2, G1, G3, X1 or X2) those phones all use CDMA SIMs. Check your coverage. Here’s a link for more on that:
If CDMA coverage is best for you, don’t try to bring a phone that is GSM-only.