From what I read I think these answers are probably right but if you are like me and dont give up easily, heres my experience had same problem as you basicaly damaged security zone my case wrong eprom k750 I know 2010 but still principle the same you need copy of unused gdfs never used in other phone. Ask Mauro. Access to service box and cable (i.e. Setool) as the phones fs probably dont exist anymore and this is also where usb, bootloader etc lived so wont detect fastport cable. Flash GDFS do not try power phone then flash main and fs and depending on what software u using either add cust pack or tick finalize or complete. Now it worked for me on a 2010 but a 2020 ? but hey whats the alternative the mantlepiece I wouldnt be able to leave it there and I know this procedure is endorsed by lasuss because hes discussed the procedure on SE-NSE and and in setool forumb why would they give access to these "virgin" GDFS files in support. Let me know how u get on and if you cant get hold of something dont hurt to ask! Good luck!