Any time you change the memory (add, remove, or just move it) on a board, you should clear the BIOS/CMOS, and let the memory start up with all Auto memory settings in the BIOS. You should try this now with your new memory, and if you are lucky it might work fine, but I cannot guarantee that. You also need to clear the BIOS just to get your old memory working correctly, when it is used by itself.
Usually when different models of memory are used together, and are incompatible with each other, or the system's memory controller, the memory that can be used will run it single channel mode.
Have you tried using the new memory only in the board, and does it work by itself?
An IOMMU is a memory mapping unit that translates a virtual address into a physical memory address. Clearing the BIOS/CMOS should clear any IOMMU tables in use, which is essential when changing a system's memory in any way. If you have an IOMMU option in your BIOS, what is its description?