I have a Samsung J3 from Republic but when I log in to Republic it says I don’t have an active phone.
The most common reason for your experience is signing into a Republic account other than the one used to activate your phone. Might you have used a different email address to set up the account used to purchase your phone than the one used to sign into Community? If that’s not it, you’ll need to raise a help ticket. You may do so here: Republic Help.
Once back into the appropriate account, the following will help with transferring your number.
I suggest starting by making sure the number in question will transfer. You may do so here: Number Transfer | Republic Wireless.
Presuming the number you wish to transfer passes the number check tool and you haven’t already done so, Republic’s documentation describing how to transfer your number and gather the needed information to do so is here: Number Transfers.
Some additional information for service providers not covered in Republic’s documentation here: Porting Guides for Transferring Your Number to Republic | Republic Wireless Wiki | Fandom powered by Wikia. Alternatively, you might contact your current service provider directly for the necessary information. Generally, they are required to provide the information you seek. See here, Porting: Keeping Your Phone Number When You Change Providers | Federal Communications Commission.