You mentioned Heimdall in the OP, are/were you aware Heimdall requires this Download mode. I wrote a few words about Heimdall + Debian here recently. You might also check out [HOWTO] (Needs expert review) Flash Galaxy s9+ to Stock Android 10 using heimdall-gui.
However with Samsung Official you will be able to ask for help here, Product Help & Support | Samsung United Kingdom This phone must have a recovery method. Edit, Reviewing this, I guess you have not actually confirmed you entered Recovery mode.