You can manipulate boot settings using BMC commands. No need to mess with Grub or the running system. Instead, tell the system to boot up from an emulated USB drive (image can be attached from some remote server, often including your web browser).

Now wait for the machine to get rebooted (or do it yourself using the BMC, e.g. 'racadm serveraction powercycle' for Dell/iDRAC machines).

Even SecureBoot won't help as you can just turn it off using the BMC.

See here for a bunch of examples for Dell machines using the BMC's HTTP API:

https://github.com/dell/iDRAC-Redfish-Scripting/tree/master/...