There are four RAM slots; two blue, two grey. It came with one 8GB
module in a blue slot. I added a second 8GB module in the other blue
slot; all ok, memory checked by Windows 10.
So, I got two new modules exactly the same as the second one, and put
them in the remaining slots. And cram!
I took those two out and booted ok. I put the first new one in one grey
slot, and still cram! Same with just the second one in that slot.

This from the Mobo User Manual;
Memory configurations
You may install 1GB, 2GB, 4GB, and 8GB unbuffered ECC, non-ECC DDR3
DIMMs into the
DIMM sockets.
• Memory module with memory frequency higher than 2133 MHz and its
corresponding
timing or the loaded DRAM OC Profile is not the JEDEC memory standard. The
stability and compatibility of these memory modules depend on the CPU’s
capabilities
and other installed devices.
• You may install varying memory sizes in Channel A and Channel B.
The system maps
the total size of the lower-sized channel for the dual-channel
configuration. Any excess
memory from the higher-sized channel is then mapped for single-channel
operation.
• We recommend that you install the memory modules from the blue
slots for better
overclocking capability.
• Always install DIMMs with the same CAS latency. For optimal
compatibility, we
recommend that you install memory modules of the same version or date
code (D/C)
from the same vendor. Check with the retailer to get the correct mem

Help, help, help!

Ed