After restored bios, look for ME service jumper, put in service mode and use the Intel FPT tool from the correct ME package tools and make a full spi dump and a bios region, restore the ME jumper, work the mods on the bios region, flash again with FPT tool the mod, if it fails you need to set the jumper back to service mode, this jumper must be set to default setting for a normal working state of the system after flashing ONLY the mod bios region.
https://winraid.level1techs.com/t/me-analyzer-intel-engine-firmware-analysis-tool-discussion
EDIT: The steps i provide still can be done when you have the machine back as original function.
No ME jumper but still can work, all depends if the tool can read/write the spi regions.
Dell system/and or specific Dell motherboards has it own way of recover the system bios, its your task to search for it on Dell documentation, the SPI programmer is the last resource when everything else fails or to froce a mod flash.