Mac won't boot of an external drive, starts internet recovery instead I wanted to clean a MacBook Pro 15,2 with Touchbar.
However I noticed that it wasn't running smoothly, so I backed it up using the Target Disk mode and erased the disk, because the Recovery partition was corrupted and didn't boot (a major red flag, that i sadly ignored). 

Next I created a Mojave install media, conected it and booted up with Alt pressed.
However when I selected the icon the Macbook pro went to start internet recovery, so I waited for it to finish. 

The bar filled up, mac rebooted and after the apple logo and bar showed up there was high fan spin and then another reboot.
Of course it didn't boot into recovery it just showed the "missing files" sign, which is not suprising. (I repeated the process several times, but nothing changed.)

Then I tried everything. Atempting to boot from anything external fails (and it doesn't see TimeMachine backups) and loads internet recovery. I flashed the "BaseSystem.dmg" from the installer to the internal drive, it shows up in the bootmanager, but no luck.

Only idea I have to put all the files from the backup back in, but who knows.

I'll be glad for any advice. Thank you.
Presumably your data on the backup is fine, it just doesn't have the correct boot sectors or they are corrupted. Why not go thru with the internet recovery, get your Macbook to a base state, then you can use Migration Assistant to move files from your backup to the now refreshed Macbook.