Samsung S6 edge plus two phones - same problem

Hello, I've got a Samsung S6 edge plus that has a motherboard problems, it seems. I've purchased another one from eBay to replace the motherboards but it looks like it has the same problem. Please help and advise if there is a problem with this series of samsung's phones or what. The problem is as follows:

1. phone only boots to Samsung S6 edge plus logo and it reboots.
2. I can only enter DOWNLOAD MODE (no recovery mode possible)
3. I have tried flashing stock firmware (couple of different countries versions) and ODIN procedure goes all rights (it says "PASSED") but the problem remains.

There is nothing else that comes to mind. Please help, I can send video if necessary, I am tech savvy and I've opened and repaired quite a few phones from Samsung. But this looks like I might need something like Octopus Box or? Thanks