I don't know what you are doing wrong; anyone can reply to a post. Just above the banner at the bottom of the screen is a spot where it says 'Reply...'. Click there, type your response in the box that appears and then click on the Post Reply button.
As for your situation, if the memory came with the system, then I would be contacting Dell and having them replace this memory. If that doesn't work, I would be having them replace the system. You paid them for a working system with memory running at a specific speed and you should get that.
I note that, on the Dell page, they list the memory as being shipped running at 3600MHz. If this memory is capable of 4800MHz, it should run at 4800MHz. If it cannot, there is something wrong with the system - and again you are back to contacting Dell for a replacement. I suspect that they never validated the solution before shipping it.