precision 3630 tower memory

Hi I have purchased a Precision 3630 tower with a four core processor. reading the manual it say a core system should have 2400 mhz memory, but when looking at the bios screen the 8 gig memory (non ECC) the system came with and is happy working with shows as 2666 mhz. As I wanted to to up to 32 gig of memory and convert to ECC memory I ordered 2 times 16 gig DDR4 2400 ECC -R modules, when these modules are fitted I only get Flashing start led code of 2 then 7 which the manual says is a memory - Memory SPD ( which I assume is memory speed, but as I cannot get into the bios I do not know if the speed has changed. As I supposed to change the memory speed in the bios if so how. I have since the box arrived I have carried out all the software updates from the dell update screen so should be on 1.2.0 (but I forgot to look before starting this query. thanks for any help Alan