Pull the BIOS chip and reprogram it separately, then solder it back to the card.
how?
Well, you have to find the bios chip on the card (it will most likely be a SO8 chip, though just google the part numbers and you will find an EEPROM). You should have two such chips on the card (most likely, as the BIOS is separate for each GPU).
You will the need to desolder the chip (which is not fun for SMD chips at all), there are some guides on how to do that without destroying the chip.
Then you will need to connect that chip to a programmer (in a pinch I guess a Raspberry Pi could be modified to work as a slow programmer, but ideally you would need an universal programmer (read the description to see if it supports your chip).
Then you need to erase the chip and write the proper BIOS to it (using the software provided with the programmer) and solder it back to the video card.
Yes, it's not fun at all, but if you manage to brick a device by writing the wrong firmware, this is what you need to do. While your card may have a JTAG and/or ISP, it may use some proprietary codes to do it and you won't know the pinout too. Though if someone has a way to use the ISP or JTAG port for your card, then you can use that to reprogram the chip.
