Hello! You could try downloading the stock firmware from a site like SamMobile and then flashing it to the phone using Odin, samsung's in-house tool, after booting into download mode by holding power and volume down until the warning screen comes up, where you can then press the up volume button to continue to download mode. Unfortunately this method results in the loss of all your data and requires a PC, but it may be your best bet! @jakellike