Help with RAM selection

Hopefully someone here can point me in the right direction. I currently have a X10SL7-F with 4 x HMT41GU7AFR8C-RD - SK Hynix 1x 8GB DDR3-1866 ECC UDIMM PC3-14900E Dual Rank x8 Module.

I have just bought a X10SAE. On the Supermicro website it says the board is compatible with up to 32GB Unbuffered ECC/non-ECC UDIMM, DDR3-1600MHz. The only difference I could see was the 1866MHz so I thought I would give it a go anyway as in the supported memory types on the Supermicro website it says it does support 1866MHz. When I switch the motherboard on I get 5 short beeps and 1 long beep which according to the manual means ‘No memory detected’.

Can someone point me in the right direction, I believe I need to order new ram but because I don’t fully understand the problem I don’t want to order the wrong ram. I am based in the UK if someone has a product suggestion.