@robimarko - I am now successfully using a version that is a hybrid of the 18.06.2 rpi3b+ specific build, and this set of files from the boot in your version:
bcm2710-rpi-3-b.dtb
bcm2710-rpi-3-b-plus.dtb
bootcode.bin
fixup_cd.dat
fixup.dat
start_cd.elf
start.elf
It turns out that some other versions of those same-named Broadcom proprietary files are incompatible with 18.06.2, e.g., the latest raspbian and raspbian-lite versions. (The software runs but only 108Mb of ram is available - a subtle fail. Described here. )
I've been searching for the Raspbian versions containing binary equivalents of your files but haven't found them.
Can you please inform exactly in which Raspian version can be found the above Broadcom proprietary files? I think that information is the best way to help others going forward.