Happy to found this post!
I have succesfully unbricked by Dahua VTH1550CH-2 by patching it with the extracted .img files from firmware General_Multi3_VTH1550CHW-2_EngRusItlFreGerDutSpaPor_P_V3.100.0000.0.R.20170401.bin.
In general I have followed the instructions above -> but has cost me...