Hello, thanks for choosing Dell.

We checked your memory.

What you need to do is to follow this guideline.

https://www.dell.com/support/manuals/en-bz/poweredge-r350/r350_ism_pub/general-memory-module-installation-guidelines?guid=guid-0d5fa0b4-6dbf-4290-8c42-c4f1e25f1f18&lang=en-us

General memory module installation guidelines

• All DIMMs must be DDR4.
• If memory modules with different speeds are installed, they operate at the speed of the slowest installed memory module.
• Populate memory module sockets only if a processor is installed.
• For single-processor systems, sockets A1 to A4 is available.
• In Optimizer Mode, the DRAM controllers operate independently in the 64-bit mode and provide optimized memory performance.

This video would also help:

https://www.youtube.com/watch?v=eXkmE0rtYW4

Dell PowerEdge R350 Server Memory Overview & Upgrade | How to Install Memory | Supported DIMMs