Try hitting the DEL key right when the keyboard lights up...keep hitting it a few times right when the lights come on.
The website claims the 5800x has been supported since August of 2020...how old is your motherboard ?
If you bought it this year I would think the BIOS support is there for the 5800x but it may be worth your while to borrow a 3000 series CPU and give it a try. If you don't have a 3000 series chip available I would call around to some local pc shops and see if they have one they could use to flash the BIOS with.