It says activation underway for a minute and then goes back to the “unsupported phone” message.
If the SIM card you have is from an RW branded phone or from a phone whose service was discontinued then it won’t work. The RW branded phones used the SIM card only to access LTE data, not to connect to the voice network. If it was used for a line that was discontinued it cannot be used in another phone.
If it is a new SIM card from RW then your phone needs to match the specifications below (from this document – Republic Wireless 3.0 FAQs) :
It has to be a particular model, SM-G920T, and have one of the following build numbers:
For Samsung Galaxy S6
Not sure if T-Mo can/will help you get a phone with the correct software build if it doesn’t already have it installed.
If you have the right model and one of the correct builds then you may want to contact support for assistance. Here is the Help document.
Special Note: The T-Mobile branded version is NOT compatible at this time.
Above info can be found on the BYOP page. Click on “Check compatibility without installing the app” in blue.