Dell bios has a hard time approving the firmware of new amd gpu based on its current settings.

try use old gpu to enter bios, disable secure boot in bios.  If it is already off, try turn it on.  save changes then exit.

if still no luck, replace cmos battery with a new battery and clear cmos settings via motherboard jumper.

there is a slight possibility that the 6750 xt you had has issue if after all you still get stuck in post which is when Alienhead stuck on screen.  6750 xt should be c/w R7 as a very close model 6700 xt is validated in R7.  https://www.userbenchmark.com/UserRun/57058579

Thanks for the assist.