With the help of Dell engineer we found the solution. My rams are: Samsung 64GB (32GB x 2) DDR4 PC4-25600 3200MHZ SODIMM and Hynix 64GB (32GB x 2) DDR4 PC4-25600 3200MHz SODIMM. Using the 4 SODIMM slots (DIM A to D) you will have a speed of 2933Mhz by combining these rams as follow 1 xSamsung+1 Hynix (DIM A+DIMC)1 xSamsung+1 Hynix (DIM B+DIMD)
Hi, My Dell Precision 7550, had an initial memory of 64Gb (2 x32GB) with a speed of 2933Mhz. I  upgraded to 128GB of DDR4-3200MHz RAM by adding 64GB (2x32GB) and the reported RAM speed is now  only 2133MHz. In Dell 7550 Septup and Specifications guide the speeds are 2666 MHz, 2933 MHz, 3200 MHz. Any advice or support is welcome.  
Dell Precision 7550 memory speed 2133 Mhz